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THE CHEMICAL ELEMENTS AND THEIR COMBINATIONS.
Thelimits of the present Work allow only of a simple sketch of the subjects which it is proposed to treat in this Chapter. Our attention therefore must be confined to an explanation of certain points which are alluded to in the First Part of the Work, and without a proper understanding of which it will be impossible for the reader to make progress.
The following division may be adopted:—The more important Elementary Bodies, with their symbols and atomic weights; the Compounds formed by their union; the class of Salts; illustrations of the nature of Chemical Affinity; Chemical Nomenclature; Symbolic Notation; the laws of Combination; the Atomic Theory; the Chemistry of Organic Bodies.
THE CHEMICAL ELEMENTS, WITH THEIR SYMBOLS AND ATOMIC WEIGHTS.
The class of elementary bodies embraces all those substances which cannot, in the present state of our knowledge, be resolved into simpler forms of matter.
The chemical elements are divided into "metallic" and"non-metallic," according to the possession of certain general characters.
The following are some of the principal non-metallic elements, with the symbols employed to designate them, and their atomic weights:[55]—
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The metallic elements are more numerous. The following list includes only those which are commonly known:—
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[55]The atomic weights, with the exception of that of Gold, are taken from the last edition of Brande's 'Manual of Chemistry.'
[55]The atomic weights, with the exception of that of Gold, are taken from the last edition of Brande's 'Manual of Chemistry.'
ON THE BINARY COMPOUNDS OF THE ELEMENTS.
Many of the elementary bodies exhibit a strong tendency to combine with each other, and to form compounds, which differ in properties from either of their constituent elements. This attraction, which is termed "Chemical Affinity," is exerted principally between bodies which are opposed to each other in their general characters. Thus, taking for example the elements Chlorine and Iodine—they are analogous in their reactions, and therefore there is but little attraction between them, whereas either of the two combines eagerly with Silver, which is an element of a different class. So, again. Sulphur unites with the metals, but two metallic elements are comparatively indifferent to each other.
Oxygen is by far the most important in the list of chemical elements. It combines with all the others, with the single exception, perhaps, of Fluorine. The attraction, or chemical affinity, however, which is exerted, varies much, in different cases. The metals, as a class, are easily oxidized; whilst many of the non-metallic elements, such as Chlorine, Iodine, Bromine, etc., exhibit but little affinity for Oxygen. Nitrogen is also a peculiarly negative element, showing little or no tendency to unite with the others.
Classification of binary compounds containing Oxygen.—When one simple element unites with another, the product is termed a "binary" compound.
There are three distinct classes of binary compounds of Oxygen:—Neutral Oxides, basic Oxides, and acid Oxides.
Neutral and basic Oxides.—Take as examples—the Oxide of Hydrogen, or Water, a neutral Oxide; the Oxide of Potassium, or Potash, a basic Oxide.
Water is termed a neutral oxide, because its affinities are low, and it is comparatively indifferent to other bodies. Potash and Oxide of Silver are examples of basic oxides; but there is a great difference between the two in chemicalenergy, the former belonging to a superior class of bases, viz. the alkaline.
By studying the properties of an alkali (such as Potash or Soda) which are familiar to all, we gain a correct notion of the whole class of basic oxides. An alkali is a substance readily soluble in water, and yielding a solution which has a slimy feel from its solvent action upon the skin. It immediately restores the blue colour of reddened litmus, and changes the blue infusion of cabbage to green. Lastly, it is neutralized and loses all its characteristic properties upon the addition of an acid.
Theweaker basesare, as a rule, sparingly or not at all soluble in water, neither have they the same caustic and solvent action upon the skin; but they restore the colour of reddened litmus, and neutralize acids in the same manner as the more powerful bases, or alkalies.
TheAcidOxides.—This class, taking the stronger acids as the type, may be described as follows:—very soluble in water, the solution possessing an intensely sour taste, and acorrodingrather than a solvent action upon the skin; changes the blue colour of litmus and other vegetable substances to red, and neutralizes the alkalies and basic oxides generally.
Observe however that these properties are possessed in very various degrees by different acids. Prussic Acid and Carbonic Acid, for instance, are not sour to the taste, and being feeble in their reactions, redden litmus scarcely or not at all. All acids however, without any exception, tend to combine with bases and to neutralize themselves; so that this may be said to be the most characteristic property of the class.
Chemical composition of Acid and Basic Oxides contrasted.—It is a law commonly observed, although with many exceptions, that bases are formed by the union of Oxygen withmetals; and acids, by Oxygen uniting withnon-metallic elements. Thus, Sulphuric Acid is a compound of Sulphur and Oxygen; Nitric Acid, of Nitrogenand Oxygen. But the alkali, Potash, is an oxide of themetalPotassium; and the oxides of Iron, Silver, Zinc, etc. are bases, and not acids.
Again, the composition of acids and bases is different in another respect; the former invariably contain more Oxygen in proportion to the other element than the latter. Taking the same examples as before, the two classes may be represented thus:—
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The class of Hydrogen Acids.—Oxygen is so essentially the element which forms the acidifying principle of acids, that its very name is derived from that fact (οξυς, acid, and γενναω, to generate). Still there are exceptions to this rule, and in some acidsHydrogenappears to play the same part; theHydracids, as they are termed, are formed principally by Hydrogen uniting with elements like Chlorine, Bromine, Iodine, Fluorine, etc. Thus, Muriatic or Hydrochloric Acid contains Chlorine and Hydrogen; Hydriodic Acid contains Iodine and Hydrogen.
Observe, however, that the position held by the Hydrogen in these compounds, is different from that of the Oxygen in the "Oxyacids," as regards the number of atoms usually present; thus—
so that the composition of the Hydracids is analogous to thebasicoxides, in containing a single atom of each constituent.
THE TERNARY COMPOUNDS OF THE ELEMENTS.
As the various elementary substances unite with each other to form Binary Compounds, so these binary compounds again unite and formTernaryCompounds.
Compound bodies however do not, as a rule, unite with simple elements. In illustration, take the action of Nitric Acid upon Silver, described atpage 12. No effect is produced upon the metal untilOxygenis imparted; then the Oxide of Silver so formed dissolves in the Nitric Acid. In other words, it is necessary that a binary compound should be first formed, before the solution can take place. The mutual attraction or chemical affinity exhibited by compound bodies is, as in the case of elements, most strongly marked when the two substances are opposed to each other in their general properties.
Thus,acidsdo not unite with other acids, but they combine instantly withalkalies; the two mutually neutralizing each other and forming "a salt."
Saltstherefore are ternary compounds produced by the union of acids and bases; common Salt, formed by neutralizing Muriatic Acid with Soda, being taken as the type of the whole class.
General characters of the Salts.—An aqueous solution of Chloride of Sodium, or common Salt, possesses those characters which are usually termed saline; it is neither sour nor corrosive, but, on the other hand, has a cooling agreeable taste. It produces no effect upon litmus and other vegetable colours, and is wanting in those energetic reactions which are characteristic of both acids and alkalies; hence, although formed by the union of two binary compounds, it differs essentially in properties from both.
All salts however do not correspond to this description of the properties of Chloride of Sodium. The Carbonate of Potash, for instance, is an acrid and alkaline salt, and the Nitrate of Iron reddens litmus-paper. A perfectly neutral salt is formed when a strong acid unites with an energetic base; but if, of the two constituents, one is more powerful than the other, the properties of that one are often seen in the resulting salt. Thus the Carbonate of Potash isalkalineto test-paper, because the CarbonicAcid is feeble in its reactions; but ifNitric AcidandPotashare brought together, then a Nitrate of Potash is produced, which is neutral in every sense of the term.
The Chloride of Sodium and salts of a similar kind are freely soluble in water, but all salts are not so. Some dissolve only sparingly, and others not at all. The Chloride and Iodide of Silver are examples of the latter class; they are not bitter and caustic like the Nitrate of Silver, but are perfectly tasteless from being insoluble in the fluids of the mouth.
It is seen therefore from these examples, and many others which might be adduced, that the popular notion of a saline body is far from being correct, and that, in the language of strict definition, any substance is a salt which is produced by the union of an acid with an alkali, independent of the properties it may possess.
Thus,Cyanide of Potassiumis a true salt, although highly poisonous; Nitrate of Silver is a salt; the green Sulphate of Iron is a salt; so also is Chalk or Carbonate of Lime, which has neither taste, colour, nor smell.
On the "Hydracid" class of Salts.—The distinction between Oxyacids and Hydracids has already been pointed out (p. 309), the latter having been shown to consist of Hydrogen united with elements analogous in their reactions to Chlorine, Iodine, Bromine, etc.
In a salt formed by an Oxygen Acid, both the basic and acid elements appear. Thus the common Nitre, which is a Nitrate of Potash, is found by analysis to contain Oxide of Potassium as a base, in a state of combination with Nitric Acid. But if a salt be formed by neutralizing an alkali with aHydrogen Acid, the product in that case does not contain all the elements. This is seen from the following example:—
or, stated more at length,—
Observe that the Hydrogen and Oxygen, being present in the correct proportions, unite to form Water, which is an Oxide of Hydrogen. This water passes off when the solution is evaporated, and leaves the dry crystals of salt. On the other hand, with the Oxyacid Salts, the elementary Hydrogen being absent, no water is formed, and the Oxygen remains.
It must therefore be borne in mind that salts like the Chlorides, Bromides, Iodides, etc. contain onlytwoelements; but that in the Oxyacid Salts, such as Sulphates, Nitrates, Acetates,threeare present. Thus, Nitrate of Silver consists of Nitrogen, Oxygen, and Silver, but Chloride of Silver contains simply Chlorine and metallic Silver united, without Oxygen.
The Hydracid salts however, when decomposed, yield products similar to the Oxyacid salts. For instance, if Iodide of Potassium be dissolved in water, and dilute Sulphuric Acid added, this acid, being powerful in its chemical affinities, tends to appropriate to itself the alkali; but it does not removePotassiumand liberateIodine, but takes theOxideof Potassium and sets freeHydriodic Acid, In other words, as an atom of water is produced during theformationof a Hydracid Salt, so is an atom destroyed and made to yield up its elements in thedecompositionof a Hydracid Salt.
The reaction of dilute Sulphuric Acid upon Iodide of Potassium may be stated thus:—
THE NATURE OF CHEMICAL AFFINITY FURTHER ILLUSTRATED.
Illustration from the Non-metallic Elements.—If a streamof Chlorine gas be passed into a solution containing the same salt as before mentioned, viz. the Iodide of Potassium, the result is to liberate a certain portion of Iodine, which dissolves in the liquid, and tinges it of a brown colour. The element Chlorine, possessing a degree of chemical energy superior to that of Iodine, prevails over it, and removes the Potassium with which the Iodine was previously combined.
The same Law illustrated by the Metals.—A strip of Iron dipped in solution of Nitrate of Silver becomes immediately coated with metallic Silver; but a piece of Silver-foil may be left for any length of time in Sulphate of Iron without undergoing change: the difference depends upon the fact, that metallic Iron has a greater attraction for Oxygen than Silver, and hence it displaces it from its solution.
Illustrations amongst Binary Compounds.—If a few drops of solution of Potash be added to solution of Nitrate of Silver, a brown deposit is formed, which is the Oxide of Silver, sparingly soluble in water. That is to say, as a stronger metal displacesmetallic Silver, so does an oxide of the same metal displaceOxide of Silver. Therefore bases like the alkalies, alkaline earths, etc. cannot exist in a free state in solutions of the salts of weaker bases,—a liquid containing Nitrate of Silver could not also contain free Potash or Ammonia.
In the list given atpage 306, the metallic elements are arranged principally in the order of their chemical affinities; those of Potassium, Sodium, Barium, etc. being the most marked.
As the alkalies displace the weaker bases from theircombination with acids, so the strongacidsdisplace weak acids from their combination with bases. Thus, as
So
In the list of acids. Sulphuric Acid is usually placed first as being the strongest, and Carbonic Acid, which is a gaseous substance, last. The vegetable acids, such as Acetic, Tartaric, etc., areintermediate, being weaker than the mineral acids, but stronger than Carbonic, or Hydrocyanic Acid.
The order of decompositions affected by the insolubility or the volatility of the products which may be formed.—It might be inferred from remarks already made, that on mixing saline solutions, a gradual interchange of elements would take place, until the strongest acids were associated with the strongest bases, andvice versâ. There are many causes however which interfere to prevent this; one of which isvolatility.—-
The violent effervescence which takes place on treating aCarbonateof any kind with an acid is due to thegaseousnature of Carbonic Acid and its escape in that form, which greatly facilitates the decomposition.
Insolubilityis also a cause which exercises a great influence on the result which will follow in mixing solutions. If the formation of an insoluble substance is possible by any interchange of elements, it will take place. A solution of Chloride of Sodium added to Nitrate of Silver invariably produces Chloride of Silver; theinsolubilityof Chloride of Silver being the cause which determines its formation.
So again, Sulphate of Lead and Protonitrate of Iron are produced by mixing Nitrate of Lead with Sulphate of Iron; but if Nitrate ofPotashbe substituted for Nitrateof Lead, the result is uncertain, because there are no elements present which can, by interchanging, form an insoluble salt; Sulphate of Potash, althoughsparinglysoluble in water, not beinginsoluble, like the Sulphate of Lead or the Sulphate of Baryta.
ON CHEMICAL NOMENCLATURE.
The nomenclature of the chemicalelementsis mostly independent of any rule; but an attempt has been made to obviate this in the case of those of later discovery. Thus the names of the newly-foundmetalsusually end inum, as Potassium, Sodium, Barium, Calcium, etc.; and those elements which possess analogous characters have corresponding terminations assigned to them, as Chlorine, Bromine, Iodine, Fluorine, etc.
Nomenclature of Binary Compounds.—These are often named by attaching the terminationideto the more important element of the two; as, the Oxideof Hydrogen, or Water; the Chlorideof Silver; the Sulphideof Silver. Binary compounds of Sulphur however are sometimes termed Sulphurets, as theSulphuretor theSulphideof Silver indifferently.
When the same body combines with Oxygen, or the corresponding element, in more than one proportion, the prefixprotois applied to that containing the least Oxygen;sesquito that with once and a half as much as theproto;biorbinto that with twice as much; andperto the one containing the most Oxygen of all. As examples, take the following:—The Protoxide of Iron; the Sesquioxide of Iron: the Protochloride of Mercury; the Bichloride of Mercury. In these examples the Sesquioxide of Iron is also aPeroxide, because no higher simple oxide is known, and the Bichloride of Mercury is aPerchloride for a similar reason.
When an inferior compound is discovered, it is often termedsub; as the Suboxide of Silver, the Subchloride of Silver. These bodies contain the least known quantityof Oxygen and Chlorine respectively, and are hence entitled to the prefixproto; but being of minor importance, they are excepted from the general rule.
The combinations of metallic elements with each other are termed "alloys;" or if containing Mercury, "amalgams."
Nomenclature of binary Compounds possessing acid properties.—These are named on a different principle. The terminationicis applied to one element. Thus, taking as an illustration the liquid known as "Oil of Vitriol," it is truly anOxideof Sulphur, but as it possesses strong acid properties it is termed SulphuricAcid. So Nitric Acid is an Oxide of Nitrogen; Carbonic Acid is an Oxide of Carbon, etc. When there are two oxides of the same element, both possessing acid properties, the most important has the terminationic, and the otherous; as Sulphuric Acid, SulphurousAcid; Nitric Acid, NitrousAcid.
Nomenclature of the Hydracids.—The Hydrogen Acids are distinguished from Oxyacids by retaining the names of both constituents, the terminationicbeing annexed as usual. Thus,Hydrochloric Acid, or the Chloride of Hydrogen;Hydriodic Acid, or the Iodide of Hydrogen.
Further illustrations of the nomenclature of Binary Compounds.—The Oxides of Nitrogen, and also of Sulphur, afford an interesting illustration of the principles of nomenclature. The former are as follows:—
Observe, that two only out of the five possess acid properties, the others being simple oxides. Nitric Acid is, strictly speaking, the "Peroxide," but as it belongs to the class of acids, that term naturally falls to the compound below.
The binary compounds of Sulphur with Oxygen all possess acid properties; they may be represented (in part) as follows:—
In this case the Sulphuric and Sulphurous Acids had become familiarly known before the others, intermediate in composition, were discovered. Hence, to avoid the confusion which would result from changing the nomenclature, the new bodies are termedHyposulphuric andHyposulphurous (from ὑπο,under).
Nomenclature of Salts.—Salts are named according to the acid they contain; the terminationicbeing changed intoate, andousintoite. Thus, Sulphuric Acid forms Sulphates; Nitric Acid, Nitrates; but SulphurousAcid forms Sulphites, and Nitrous Acid, Nitrites.
In naming a salt, the base is always placedafterthe acid, the termoxidebeing omitted; thus.Nitrate of Oxide of Silveris more shortly known as "Nitrate of Silver," the presence of Oxygen being understood.
When there are two oxides of the same base, both of which aresalifiable,—in naming the salts, the termprotois prefixed to the acid of the salt formed by the lowest, and per to that of the higher oxide; as, theProtosulphate of Iron, or Sulphate of the Protoxide; thePersulphate of Iron, or Sulphate of the Peroxide.
Many salts contain more than one atom of acid to each atom of base. In that case, the usual prefixes expressive of quantity are adopted: thus, theBisulphate of Potash contains twice as much Sulphuric Acid as the neutral Sulphate, etc.
On the other hand, there are salts in which the base is in excess with regard to the acid, and which are usually known as "basic salts;" thus, the red powder whichdeposits from solution of Sulphate of Iron, is abasicPersulphate of Iron, or a Sulphate of the Peroxide of Iron with more than the normal proportion of oxide.
Nomenclature of the Hydracid Salts.—The composition of these salts being different from those formed by Oxygen Acids, the nomenclature varies also. Thus, in neutralizing Hydrochloric Acid with Soda, the product formed is not known as Hydrochlorate of Soda, but asChloride of Sodium; this salt, and others of a similar constitution, beingbinary, and notternary, compounds. The salt produced by Hydrochloric Acid andAmmoniahowever is often called "Muriate or Hydrochlorate of Ammonia," although more strictly it should be theChloride of Ammonium.
ON SYMBOLIC NOTATION.
The list of symbols employed to represent the various elementary bodies is given atpage 306.—Commonly the initial letter of the Latin name is used, a second or smaller letter being added when two elements correspond in their initials: thus C stands for Carbon, Cl for Chlorine, Cd for Cadmium, and Cu for Copper.
The chemical symbol however does not simply represent a particular element; it denotes also a definite weight, or equivalent proportion, of that element. This will be explained more fully in the succeeding pages, when speaking of the Laws of Combination.
Formulæ of Compounds.—In thenomenclatureof compounds it is usual to place the Oxygen or analogous elementfirstin the case of binary compounds, and the acid before the base in the ternary compounds, or salts; but in representing themsymbolicallythis order is reversed: thus, Oxide of Silver is written AgO, and never as OAg; Nitrate of Silver as AgO NO5, not NO5AgO.
The juxtaposition of symbols expresses combination; thus, FeO is a compound of one proportion of Iron with one of Oxygen, or the "Protoxide of Iron," If more thanone equivalent be present, small figures are placed below the symbols: thus, Fe2O3represents two equivalents of Iron united with three of Oxygen, or the "Peroxide of Iron;" SO3, one equivalent of Sulphur with three of Oxygen, or Sulphuric Acid.
Larger figures placed before and in the same line with the symbols, affect thewhole compoundwhich the symbols express: thus, 2 SO3means two equivalents of Sulphuric Acid; 3 NO5, three equivalents of Nitric Acid. The interposition of a comma prevents the influence of the large figure from extending further. Thus, the double Hyposulphite of Soda and Silver is represented as follows:—
2 NaO S2O2, AgO S2O2,
ortwoequivalents of Hyposulphite of Soda with one of Hyposulphite of Silver; the large figure referring only to the first half of the formula. Sometimes brackets, etc. are employed, in order to render a complicated formula more plain. For example, the formula for the double Hyposulphite of Gold and Soda, or "Sel d'or," may be written thus;—
3 (NaO S2O2) AuO S2O2+ 4 HO.
In this formula, theplus sign(+) denotes that the four atoms of water which follow, are less intimately united with the framework of the salt than the other constituents.
The use of a plus sign is commonly adopted in representing salts which contain water of crystallization. Thus, the formula for the crystallized Protosulphate of Iron is written as follows:—
FeO SO3+ 7 HO.
These atoms of water are driven off by the application of heat, leaving a white substance, which is the Anhydrous salt, and would be written simply as FeO SO3.
Theplussign however is often employed in token of simpleaddition, no combination of any kind being intended. Thus the decomposition which follows on mixingChloride of Sodium with Nitrate of Silver may be written as follows:—
NaCl + AgO NO5= AgCl + NaO NO5;
that is,—
ON EQUIVALENT PROPORTIONS.
When elementary or compound bodies enter into chemical union with each other, they do not combine in indefinite proportions, as in the case of a mixture of two liquids, or the solution of a saline body in water. On the other hand, a certain definite weight of the one unites with an equally definite weight of the other; and if an excess of either be present, it remains free and uncombined.
Thus, if we take asingle grainof the element Hydrogen—to convert that grain into Water there will be required exactly 8 grains of Oxygen; and if a larger quantity than this were added, as for instanceten grains, then two grains would be over and above. So, to form Hydrochloric Acid, 1 grain of Hydrogen takes 36 grains of Chlorine:—for theHydriodic Acid, 1 grain of Hydrogen unites with 126 grains of Iodine.
Again, if separate portions of metallic Silver, of 108 grains each, are weighed out,—in order to convert them into Oxide, Chloride, and Iodide of Silver respectively, there would be required
Therefore it appears that 8 grains of Oxygen areequivalentto 36 grains of Chlorine and to 126 grains of Iodine, seeing that these quantities all play the same part in combining; and so it is with regard to the other elements,—to every one of them a figure can be assigned which representsthe number of parts by weight in which that element unites with others. These figures are the "equivalents" or "combining proportions," and they are denoted by thesymbolof the element. A symbol does not stand as a simple representative of an element, but as a representative ofone equivalentof an element. Thus "O" indicates 8 parts by weight of Oxygen; "Cl" one equivalent, or 36 parts by weight, of Chlorine; and so with the rest.
Observe however that these figures, termed "equivalents," do not refer to theactual numberof parts by weight, but only to theratiowhich exists between them: if Oxygen is 8, then Chlorine is 36; but if we term Oxygen 100, as some have proposed, then Chlorine would be 442·65.
In the scale of equivalents now usually adopted, Hydrogen, as being the lowest of all, is taken as unity, and the others are related to it.
Equivalents of Compounds.—The law of equivalent proportions applies to compounds as well as to simple bodies, the combining proportion of a compound being always the sum of the equivalents of its constituents. Thus Sulphur is 16, and Oxygen 8, therefore Sulphuric Acid, or SO3, equals 40. The equivalent of Nitrogen is 14, that of Nitric Acid, or NO5, is 54.
The same rule applies with regard to salts. Take for instance the Nitrate of Silver: it contains
Practical application of the Laws of Combination .—The utility of being acquainted with the law of combining proportions is obvious when their nature is understood. As bodies both unite with and replace each other in equivalents,a simple calculation shows at once how much of each element or compound will be required in a given reaction. Thus, supposing it be desired to convert 100 grains of Nitrate of Silver intoChlorideof Silver, the weight of Chloride of Sodium which will be necessary is deduced thus:—one equivalent, or 170 parts, of Nitrate of Silver, is decomposed by an equivalent, or 60 parts, of Chloride of Sodium. Therefore
as 170 : 60 :: 100 : 35·2;
that is, 35·2 grains of Salt will precipitate, in the state of Chloride, the whole of the Silver contained in 100 grains of Nitrate.
So again, in order to form the Iodide of Silver, the proportions in which the two salts should be mixed is thus shown. The equivalent of Iodide of Potassium is 166, and that of Nitrate of Silver is 170. These numbers so nearly correspond, that it is common to direct that equal weights of the two salts should be taken.
One more illustration will suffice. Supposing it be required to form 20 grains of Iodide of Silver—how much Iodide of Potassium and Nitrate of Silver must be used? One equivalent, or 166 parts, of Iodide of Potassium, will yield an equivalent, or 234 parts, of Iodide of Silver; therefore
as 234 : 166 :: 20 : 14·2.
Hence, if 14·2 grains of the Iodide of Potassium be dissolved in water, and an equivalent quantity, viz. 14·5 grains, of the Nitrate of Silver added, the yellow precipitate, when washed and dried, will weigh precisely 20 grains.
ON THE ATOMIC THEORY.
The atomic theory, originally proposed by Dalton, so much facilitates the comprehension of chemical reactions generally, that it may be useful to give a short sketch of it.
It is supposed that all matter is made up of an infinite number of minute atoms, which are elementary, and do not admit of further division. Each of these atoms possesses an actual weight, although inappreciable by our present methods of investigation. Simple atoms, by uniting with each other, formcompound atoms; and when these compounds are broken up, the elementary constituent atoms are not destroyed, but separate from each other, in possession of all their original properties.
In representing the simple atomic structure of bodies,circlesmay be used, as in the following diagram.
Fig. 1.Fig. 2Fig. 3.
Fig. 1.Fig. 2Fig. 3.
Fig. 1 is a compound atom of Sulphuric Acid, consisting of an atom of Sulphur united intimately with three of Oxygen; fig. 2 is an atom of Peroxide of Nitrogen, NO4; and fig. 3, an atom of Nitric Acid, composed of Nitrogen 1 atom. Oxygen 5 atoms, or in symbols NO5.
The term "atomic weight" substituted for equivalent proportion.—If we suppose that the simple atoms of different kinds of matterdiffer in weight, and that this difference is expressed by their equivalent numbers, the whole laws of combination follow by the simplest reasoning. It is easy to understand that an atom of one element, or compound, would displace, or be substituted for, a single atom of another; therefore, taking as the illustration the decomposition of Iodide of Potassium by Chlorine,—the weight of the latter element required to liberate 126 grains of Iodine is 36 grains, becausethe weights of the atoms of those two elementary bodies are as 36 to 126. So again,in the reaction between Chloride of Sodium and Nitrate of Silver, a compound atom of the former, represented by the weight 60, reacts upon a compound atom of the latter, which equals 170.
Therefore in place of the term "equivalent" or "combining proportion," it is more usual to employ that of "atomic weight." Thus the atomic weight of Oxygen is 8, represented by the symbol O; that of Sulphur is 16; hence the atomic weight of the compound atom of Sulphuric Acid, or SO3, is necessarily equal to the combined weights of the four simple atoms;id est, 16 + 24 = 40.
ON THE CHEMISTRY OF ORGANIC SUBSTANCES.
By "organic" substances are meant those which have possessedlife, with definite organs and tissues, in contra-distinction to the various forms of dead inorganic matter, in which no structural organization of that kind is found.
The term organic however is also applied to substances which are obtained by chemical processes from the vegetable and animal kingdoms, although they cannot themselves be said to be living bodies; thus Acetic Acid, procured by the distillation of woody fibre, and Alcohol, by fermentation from sugar, are strictly organic substances.
The class of organic bodies embraces a great variety of products; which, like inorganic Oxides, may be divided into neutral, acid, and basic.
The organicacidsare numerous, including Acetic Acid, Tartaric, Citric, and a variety of others.
Theneutral substancescannot easily be assimilated to any class of inorganic compounds; as examples, take Starch, Sugar, Lignine, etc.
Thebasesare also a large class. They are mostly rare substances, not familiarly known: Morphia, obtained from Opium; Quinia, from Quinine; Nicotine, from Tobacco, are illustrations.
Composition of organic and inorganic bodies contrasted.—There are more than fifty elementary substances found in the inorganic kingdom, but onlyfour, commonly speaking, in the organic: these four are Carbon, Hydrogen, Nitrogen, and Oxygen.
Some organic bodies,—oil of turpentine, naphtha, etc., contain only Carbon and Hydrogen; many others, such as sugar, gum, alcohol, fats, vegetable acids—Carbon, Hydrogen, and Oxygen. TheNitrogenous bodies, so called, containing Nitrogen in addition to the other elements, are principally substances derived from animal and vegetable tissues, such as Albumen, Caseine, Gelatine, etc.; Sulphur and Phosphorus are also present in many of the Nitrogenous bodies, but only to a small extent.
Organic substances, although simple as regards thenumberof elements involved in their formation, are often highly complex in the arrangement of the atoms; this may be illustrated by the following formulæ:—
Inorganic bodies, as already shown, unitein pairs,—two elements join to form a binary compound; two binary compounds produce a salt; two salts associated together form a double salt. With organic bodies however the arrangement is different,—the elementary atoms are all grouped equally in one compound atom, which is highly complex in structure, and cannot be split up into binary products.
Observe also, as characteristic of Organic Chemistry, the apparent similarity in composition between bodies which differ widely in properties. As examples takeLignine, or cotton fibre, and Starch,—each of which contains the three elements united as C24H20O20.
Mode of distinguishing between Organic and Inorganic matter.—A simple means of doing this is as follows:—place the suspected substance upon a piece of Platinum-foil, and heat it to redness with a spirit-lamp: if it firstblackens, and then burns completely away, it is probably of organic origin. This test depends upon the fact, that the constituent elements of organic bodies are all either themselves volatile, or capable of forming volatile combinations with Oxygen. Inorganic substances, on the other hand, are often unaffected by heat, or, if volatile, are dissipated without previous charring.
The action of heat upon organic matter may further be illustrated by the combustion of coal or wood in an ordinary furnace;—first, an escape of Carbon and Hydrogen, united in the form of volatile gaseous matter, takes place, leaving behind a black cinder, which consists of Carbon and inorganic matter combined; afterwards this Carbon burns away into Carbonic Acid, and a grey ash is left which is composed of inorganic salts, and is indestructible by heat.
VOCABULARY OF PHOTOGRAPHIC CHEMICALS.
ACETIC ACID.
Symbol, C4H3O3+ HO. Atomic weight, 60.
AceticAcid is a product of theoxidationof Alcohol. Spirituous liquids, when perfectly pure, are not affected by exposure to air; but if a portion of yeast, or Nitrogenous organic matter of any kind, be added, it soon acts as aferment, and causes the spirit to unite with oxygen derived from the atmosphere, and so to become sour from formation of Acetic Acid, or "vinegar."
Acetic Acid is also produced on a large scale by heatingwoodin close vessels: a substance distils over which is Acetic Acid contaminated with empyreumatic and tarry matter; it is termed Pyroligneous Acid, and is much used in commerce.
The most concentrated Acetic Acid may be obtained by neutralizing common vinegar with Carbonate of Soda, and crystallizing out the Acetate of Soda so formed; this Acetate of Soda is then distilled with Sulphuric Acid, which removes the Soda and liberates Acetic Acid: the Acetic Acid being volatile, distils over, and may be condensed.
Properties of Acetic Acid.—The strongest acid contains only a single atom of water; it is sold under the nameof "Glacial Acetic Acid," so called from its property of solidifying at a moderately low temperature. At about 50° the crystals melt, and form a limpid liquid of pungent odour and a density nearly corresponding to that of water; the specific gravity of Acetic Acid however is no test of its real strength, which can only be estimated by analysis.
The commercialGlacialAcetic Acid is often diluted with water, which may be suspected if it does not solidify during the cold winter months. Sulphurous and Hydrochloric Acids are also common impurities. They are injurious in Photographic Processes, from their property of precipitating Nitrate of Silver. To detect them proceed as follows:—dissolve a small crystal of Nitrate of Silver in a few drops of water, and add to it about half a drachm of the Glacial Acid; the mixture should remain quite clear even when exposed to the light. Hydrochloric and Sulphurous Acid produce a white deposit of Chloride or Sulphite of Silver; and ifAldehydeor volatile tarry matter be present in the Acetic Acid, the mixture with Nitrate of Silver, although clear at first, becomes discoloured by the action of light.
Glacial Acetic Acid sometimes has a smell of garlic. In this state it probably contains an organic Sulphur Acid, and is unfit for use.
Many employ a cheaper form of Acetic Acid, sold by druggists as "Beaufoy's" acid; it should be of the strength of the Acetic Acid fortiss. of the London Pharmacopœia, containing 30 per cent, real acid. It will be advisable to test it for Sulphuric Acid (see Sulphuric Acid), and other impurities, before use.
ACETATE OF SILVER.SeeSilver, Acetate of.
ALBUMEN.
Albumen is an organic principle found both in the animal and vegetable kingdom. Its properties are beststudied in thewhite of egg, which is a very pure form of Albumen.
Albumen is capable of existing in two states; in one of which it is soluble, in the other insoluble, in water. The aqueous solution of the soluble variety gives a slightly alkaline reaction to test-paper; it is somewhat thick and glutinous, but becomes more fluid on the addition of a small quantity of an alkali, such as Potash or Ammonia.
Soluble Albumen may be converted into theinsolubleform in the following ways:—
1.By the application of heat.—A moderately strong solution of Albumen becomes opalescent and coagulates on being heated to about 150° Fahrenheit, but a temperature of 212° is required if the liquid is very dilute. A layer ofdriedAlbumen cannot easily be coagulated by the mere application of heat.
2.By addition of strong acids.—Nitric Acid coagulates Albumen perfectly without the aid of heat. Acetic Acid however acts differently, appearing to enter into combination with the Albumen, and forming a compound soluble in warm water acidified by Acetic Acid.
3.By the action of metallic salts.—Many of the salts of the metals coagulate Albumen completely. Nitrate of Silver does so; also the Bichloride of Mercury. Ammoniacal Oxide of Silver however does not coagulate Albumen.
The white precipitate formed on mixing Albumen with Nitrate of Silver is a chemical compound of the animal matter with Protoxide of Silver. This substance, which has been termed Albuminate of Silver, is soluble in Ammonia and Hyposulphite of Soda; but after exposure to light, or heating in a current of Hydrogen gas, it assumes a brick-red colour, being probably reduced to the condition of an organic compound of aSuboxideof Silver. It is then almost insoluble in Ammonia, but enough dissolves to tinge the liquid wine-red. Thered colorationof solution of Nitrate of Silver employed in sensitizing theAlbuminized photographic paper is probably produced by the same compound, although, often referred to the presence of Sulphuret of Silver.
Albumen also combines with Lime and Baryta. When Chloride of Barium is used with Albumen, a white precipitate of this kind usually forms.
Chemical composition of Albumen.—Albumen belongs to theNitrogenousclass of organic substances (seepage 325). It also contains small quantities of Sulphur and Phosphorus.
ALCOHOL.
Symbol, C4H6O2. Atomic weight, 46.
Alcohol is obtained by the careful distillation of any spirituous or fermented liquor. If wine or beer be placed in a retort, and heat applied, the Alcohol, being more volatile than water, rises first, and is condensed in an appropriate receiver; a portion of the vapour of water however passes over with the Alcohol, and dilutes it to a certain extent, forming what is termed "Spirits of Wine." Much of this water may be removed by redistillation from Carbonate of Potash, in the manner described at page 196 of this work; but in order to render the Alcohol thoroughlyanhydrous, it is necessary to employQuicklime, which possesses a still greater attraction for water. An equal weight of this powdered lime is mixed with strong Alcohol of ·823, and the two are distilled together.
Properties of Alcohol.—Pure anhydrous Alcohol is a limpid liquid, of an agreeable odour and pungent taste; sp. gr. at 60°, ·794. It absorbs vapour of water, and becomes diluted by exposure to damp air; boils at 173° Fahr. It has never been frozen.
Alcohol distilled from Carbonate of Potash has a specific gravity of ·815 to ·823, and contains 90 to 93 per cent, of real spirit.
The specific gravity of ordinary rectified Spirits of Wineis usually about ·840, and it contains 80 to 83 per cent, of absolute Alcohol.
AMMONIA.
Symbol, NH3or NH4O. Atomic weight, 17.
The liquid known by this name is an aqueous solution of the volatile gas Ammonia. Ammoniacal gas contains one atom of Nitrogen combined with three of Hydrogen: these two elementary bodies exhibit no affinity for each other, but they can be made to unite under certain circumstances, and the result is Ammonia.
Properties of Ammonia.—Ammoniacal gas is soluble in water to a large extent; the solution possessing those properties which are termed alkaline (seepage 308). Ammonia however differs from the other alkalies in one important particular—it is volatile: hence the original colour of turmeric-paper affected by Ammonia is restored on the application of heat. Solution of Ammonia absorbs Carbonic Acid rapidly from the air, and is converted into Carbonate of Ammonia; it should therefore be preserved in stoppered bottles. Besides Carbonate, commercial Ammonia often contains Chloride of Ammonium, recognized by the white precipitate given by Nitrate of Silver after acidifying with pure Nitric Acid.
The strength of commercial Ammonia varies greatly; that sold for pharmaceutical purposes under the name of Liquor Ammoniæ, contains about 10 per cent, of real Ammonia. The sp. gr. of aqueous Ammonia diminishes with the proportion of Ammonia present, the Liquor Ammoniæ being usually about ·936.
Ammonia, although forming a large class of salts, appears at first sight to contrast strongly in composition with the alkalies proper, such as Potash and Soda. Mineral bases generally areprotoxides of metals, as already shown atpage 308, but Ammonia consists simply of Nitrogen and Hydrogen united without Oxygen. The followingremarks may perhaps tend somewhat to elucidate the difficulty:—
Theory of Ammonium.—This theory supposes the existence of a substance possessing the properties of ametal, but differing from metallic bodies generally in beingcompoundin structure: the formula assigned to it is NH4, one atom of Nitrogen united with four of Hydrogen. This hypothetical metal is termed "Ammonium;" and Ammonia, associated with an atom of water, may be viewed as itsOxide, for NH3+ HO plainly equals NH4O. Thus, as Potash is the Oxide ofPotassium, so Ammonia is the Oxide ofAmmonium.
The composition of thesaltsof Ammonia is on this view assimilated to those of the alkalies proper. Thus, Sulphate of Ammonia is a Sulphate of the Oxide of Ammonium; Muriate or Hydrochlorate of Ammonia is a Chloride of Ammonium, etc.
AMMONIO-NITRATE OF SILVER.SeeSilver, Ammonio-Nitrate of.
AQUA-REGIA.SeeNitro-Hydrochloric Acid.
BARYTA, NITRATE OF.SeeNitrate of Baryta.
BICHLORIDE OF MERCURY.SeeMercury, Bichloride of.
BROMINE.
Symbol, Br. Atomic weight, 78.
This elementary substance is obtained from the uncrystallizable residuum of sea-water, termedbittern. It exists in the water in very minute proportion, combined with Magnesium in the form of a soluble Bromide of Magnesium.
Properties.—Bromine is a deep reddish-brown liquid ofa disagreeable odour, and fuming strongly at common temperatures; sparingly soluble in water (1 part in 23, Löwig), but more abundantly so in Alcohol, and especially in Ether. It is very heavy, having a specific gravity of 3·0.
Bromine is closely analogous to Chlorine and Iodine in its chemical properties. It stands on the list intermediately between the two; its affinities being stronger than those of Iodine, but weaker than Chlorine (see Chlorine).
It forms a large class of salts, of which the Bromides of Potassium, Cadmium, and Silver are the most familiar to Photographers.
BROMIDE OF POTASSIUM.
Symbol, KBr. Atomic weight, 118.
Bromide of Potassium is prepared by adding Bromine to Caustic Potash, and heating the product, which is a mixture of Bromide of Potassium and Bromate of Potash, to redness, in order to drive off the Oxygen from the latter salt. It crystallizes in anhydrous cubes, like the Chloride and Iodide of Potassium; it is easily soluble in water, but more sparingly so in Alcohol; it yields red fumes of Bromine when acted upon by Sulphuric Acid.
BROMIDE OF SILVER.SeeSilver, Bromide of.
CARBONATE OF SODA.
Symbol, NaO CO2+ 10 Aq.
This salt was formerly obtained from the ashes of seaweeds, but is now more economically manufactured on a large scale from common salt. The Chloride of Sodium is first converted into Sulphate of Soda, and afterwards the Sulphate into Carbonate of Soda.
Properties.—The perfect crystals contain ten atoms of water, which are driven off by the application of heat, leaving a white powder—the anhydrous Carbonate.CommonWashing Sodais a neutral Carbonate, contaminated to a certain extent with Chloride of Sodium and Sulphate of Soda. The Carbonate used for effervescing draughts is either a Bicarbonate with 1 atom of water, or a Sesquicarbonate, containing about 40 per cent, of real alkali; it is therefore nearly double as strong as the washing Carbonate, which contains about 22 per cent, of Soda. Carbonate of Soda is soluble in twice its weight of water at 60°, the solution being strongly alkaline.
CARBONATE OF POTASH. SeePotash, Carbonate of.
CASEINE.SeeMilk.
CHARCOAL, ANIMAL.
Animal Charcoal is obtained by heating animal substances, such as bones, dried blood, horns, etc., to redness, in close vessels, until all volatile empyreumatic matters have been driven off, and a residue of Carbon remains. When prepared from bones it contains a large quantity of inorganic matter in the shape of Carbonate and Phosphate of Lime, the former of which producesalkalinityin reacting upon Nitrate of Silver (seep. 89). Animal Charcoal is freed from these earthy salts by repeated digestion in Hydrochloric Acid; but unless very carefully washed it is apt to retain an acid reaction, and so to liberate free Nitric Acid when added to solution of Nitrate of Silver.
Properties.—Animal Charcoal, when pure, consists, solely of Carbon, and burns away in the air without leaving any residue: it is remarkable for its property of decolorizing solutions; the organic colouring substance being separated, but not actuallydestroyed, as it is byChlorineemployed as a bleaching agent. This power of absorbing colouring matter is not possessed in an equal degree by all varietiesof Charcoal, but is in great measure peculiar to those derived from the animal kingdom.
CHINA CLAY, OR KAOLIN.
This is prepared, by careful levigation, from mouldering granite and other disintegrated felspathic rocks. It consists of theSilicate of Alumina,—that is, of Silicic Acid orFlint, which is an Oxide of Silicon, united with the base Alumina (Oxide of Aluminum). Kaolin is perfectly insoluble in water and acids, and produces no decomposition in solution of Nitrate of Silver. It is employed by Photographers to decolorize solutions of Nitrate of Silver which have become brown from the action of Albumen or other organic matters.
Commercial Kaolin may contain chalk, in which state it produces alkalinity in solution of Nitrate of Silver. The impurity, detected by its effervescence with acids, is removed by washing the Kaolin in diluted vinegar and subsequently in water.
CHLORINE.
Symbol, Cl. Atomic weight, 36.
Chlorine is a chemical element found abundantly in nature, combined with metallic Sodium in the form of Chloride of Sodium, or Sea-salt.
Preparation.—By distilling common Salt with Sulphuric Acid, Sulphate of Soda and Hydrochloric Acid are formed. Hydrochloric Acid contains Chlorine combined with Hydrogen; by the action of nascent Oxygen (see Oxygen), the Hydrogen may be removed in the form of water, and the Chlorine left alone.
Properties.—Chlorine is a greenish-yellow gas, of a pungent and suffocating odour; soluble to a considerable extent in water, the solution possessing the odour and colour of the gas. It is nearly 2½ times as heavy as a corresponding bulk of atmospheric air.
Chemical properties.—Chlorine belongs to a small natural group of elements which contains also Bromine, Iodine, and Fluorine. They are characterized by having a strong affinity for Hydrogen, and also for the metals; but are comparatively indifferent to Oxygen. Many metallic substances actually undergocombustionwhen projected into an atmosphere of Chlorine, the union between the two taking place with extreme violence. The characteristic bleaching properties of Chlorine gas are explained in the same manner:—Hydrogen is removed from the organic substance, and in that way the structure is broken up and the colour destroyed.
Chlorine is more powerful in its affinities than either Bromine or Iodine. The salts formed by these three elements are closely analogous in composition and often in properties. Those of the Alkalies, Alkaline Earths, and many of the Metals, are soluble in water; but the Silver salts are insoluble; the Lead salts sparingly so.
The combinations of Chlorine, Bromine, Iodine, and Fluorine, with Hydrogen, are acids, and neutralize Alkalies in the usual, manner, with formation of Alkaline Chloride and water (seepage 311).
The test by which the presence of Chlorine is detected, either free or in combination with bases, isNitrate of Silver; it gives a white curdy precipitate of Chloride of Silver, insoluble in Nitric Acid, but soluble in Ammonia. The solution of Nitrate of Silver employed as the test must not contain Iodide of Silver, as this compound is precipitated by dilution.
CHLORIDE OF AMMONIUM.
Symbol, NH4Cl. Atomic weight, 54.
This salt, also known as Muriate or Hydrochlorate of Ammonia, occurs in commerce in the form of colourless and translucent masses, which are procured bysublimation, the dry salt being volatile when strongly heated. Itdissolves in an equal weight of boiling, or in three parts of cold water. It contains more Chlorine in proportion to the weight used than Chloride of Sodium, the atomic weights of the two being as 54 to 60.
CHLORIDE OF BARIUM.
Symbol, BaCl + 2 HO. Atomic weight, 123.
Barium is a metallic element very closely allied to Calcium, the elementary basis of Lime. The Chloride of Barium is commonly employed as a test for Sulphuric Acid, with which it forms an insoluble precipitate of Sulphate of Baryta. It is also said to affect the colour of the Photographic image when used in preparing Positive paper, which may possibly be due to a chemical combination of Baryta with Albumen; but it must be remembered that this Chloride, from its high atomic weight, contains less Chlorine than the alkaline Chlorides (seepage 124).
Properties of Chloride of Barium.—Chloride of Barium occurs in the form of white crystals, soluble in about two parts of water, at common temperature. These crystals contain two atoms of water of crystallization, which are expelled at 212°, leaving the anhydrous Chloride.
CHLORIDE OF GOLD. SeeGold, Chloride of.