R2O; salts RX, hydroxides ROH. Generally basic like K2O, Na2O, Hg2O, Ag2O, Cu2O; if there are acid oxides of this composition they are very rare, are only formed by distinctly acid elements, and even then have only feeble acid properties; for example, Cl2O and N2O.R2O2or RO; salts RX2, hydroxides R(OH)2. The most simple basic salts R2OX2or R(OH)X; for instance, the chloride Zn2OCl2; also an almost exclusively basic type; but the basic properties are more feebly developed than in the preceding type. For example, CaO, MgO, BaO, PbO, FeO, MnO, &c.R2O3; salts RX3, hydroxides R(OH)3, RO(OH), the most simple basic salts ROX, R(OH)X3. The bases are feeble, like Al2O3, Fe2O3, Tl2O3, Sb2O3. The acid properties are also feebly developed; for instance, in B2O3; but with the non-metals the properties of acids are already clear; for instance, P2O3, P(OH)3.R2O4or RO2; salts RX4or ROX2, hydroxides R(OH)4, RO(OH)2. Rarely bases (feeble), like ZrO2, PtO2; more often acid oxides; but the acid properties are in general feeble, as in CO2, SO2, SnO2. Many intermediate oxides appear in this and the preceding and following types.R2O5; salts principally of the types ROX3, RO2X, RO(OH)3, RO2(OH), rarely RX5. The basic character (X, a halogen, simple or complex; for instance, NO3, Cl, &c.) is feeble; the acid character predominates, as is seen in N2O5, P2O5, Cl2O5; then X = OH, OK, &c., for example NO2(OK).R2O6or RO3; salts and hydroxides generally of the type RO2X2, RO2(OH)2. Oxides of an acid character, as SO3, CrO3, MnO3. Basic properties rare and feebly developed as in UO3.R2O7; salts of the form RO3X, RO3(OH), acid oxides; for instance, Cl2O7, Mn2O7. Basic properties as feebly developed as the acid properties in the oxides R2O.R2O8or RO4. A very rare type, and only known in OsO4and RuO4.
R2O; salts RX, hydroxides ROH. Generally basic like K2O, Na2O, Hg2O, Ag2O, Cu2O; if there are acid oxides of this composition they are very rare, are only formed by distinctly acid elements, and even then have only feeble acid properties; for example, Cl2O and N2O.
R2O2or RO; salts RX2, hydroxides R(OH)2. The most simple basic salts R2OX2or R(OH)X; for instance, the chloride Zn2OCl2; also an almost exclusively basic type; but the basic properties are more feebly developed than in the preceding type. For example, CaO, MgO, BaO, PbO, FeO, MnO, &c.
R2O3; salts RX3, hydroxides R(OH)3, RO(OH), the most simple basic salts ROX, R(OH)X3. The bases are feeble, like Al2O3, Fe2O3, Tl2O3, Sb2O3. The acid properties are also feebly developed; for instance, in B2O3; but with the non-metals the properties of acids are already clear; for instance, P2O3, P(OH)3.
R2O4or RO2; salts RX4or ROX2, hydroxides R(OH)4, RO(OH)2. Rarely bases (feeble), like ZrO2, PtO2; more often acid oxides; but the acid properties are in general feeble, as in CO2, SO2, SnO2. Many intermediate oxides appear in this and the preceding and following types.
R2O5; salts principally of the types ROX3, RO2X, RO(OH)3, RO2(OH), rarely RX5. The basic character (X, a halogen, simple or complex; for instance, NO3, Cl, &c.) is feeble; the acid character predominates, as is seen in N2O5, P2O5, Cl2O5; then X = OH, OK, &c., for example NO2(OK).
R2O6or RO3; salts and hydroxides generally of the type RO2X2, RO2(OH)2. Oxides of an acid character, as SO3, CrO3, MnO3. Basic properties rare and feebly developed as in UO3.
R2O7; salts of the form RO3X, RO3(OH), acid oxides; for instance, Cl2O7, Mn2O7. Basic properties as feebly developed as the acid properties in the oxides R2O.
R2O8or RO4. A very rare type, and only known in OsO4and RuO4.
It is evident from the circumstance that in all the higher types theacid hydroxides(for example, HClO4, H2SO4, H3PO4) and salts with a single atom of one element contain, like the higher saline type RO4,not more than four atoms of oxygen; that the formation of the saline oxides is governed by a certain common principle which is best looked for in the fundamental properties of oxygen, and in general of the most simple compounds. The hydrate of the oxide RO2is of the higher type RO22H2O = RH4O4= R(HO)4. Such, for example, is the hydrate of silica and the salts (orthosilicates) corresponding with it, Si(MO)4. The oxide R2O5, corresponds with the hydrate R2O53H2O = 2RH3O4= 2RO(OH)3. Such is orthophosphoric acid, PH3O3. The hydrate of the oxide RO3is RO3H2O = RH2O4= RO2(OH)2—for instance, sulphuric acid. The hydrate corresponding to R2O7is evidently RHO = RO3(OH)—for example, perchloric acid. Here, besides containing O4, it must further be remarked thatthe amount of hydrogen in the hydrate is equal to the amount of hydrogen in the hydrogen compound. Thus silicon gives SiH4and SiH4O4, phosphorus PH3and PH3O4, sulphur SH2and SH2O4, chlorine ClH and ClHO4. This, if it does not explain, at least connects in a harmonious and general system the fact thatthe elements are capable of combining with a greater amount of oxygen, the less the amount of hydrogen which they are able to retain. In this the key to the comprehension of all further deductions must be looked for, and we will therefore formulate this rule in general terms. An element R gives a hydrogen compound RHn, the hydrate of its higher oxide will be RHnO4, and therefore the higher oxide will contain 2RHnO4-nH2O = R2O8 -n. For example, chlorine gives ClH, hydrate ClHO4, and the higher oxide Cl2O7. Carbon gives CH4and CO2. So also, SiO2and SiH4are the higher compounds of silicon with hydrogen and oxygen, like CO2and CH4. Here the amounts of oxygen and hydrogen are equivalent. Nitrogen combines with a large amount of oxygen, forming N2O5, but, on the other hand, with a small quantity of hydrogen in NH3.The sum of the equivalents of hydrogen and oxygen, occurring in combination with an atom of nitrogen, is, as always in the higher types, equal toeight. It is the same with the other elements which combine with hydrogen and oxygen. Thus sulphur gives SO3; consequently, six equivalents of oxygen fall to an atom of sulphur, and in SH2two equivalents of hydrogen. The sum is again equal to eight. The relation between Cl2O7and ClH is the same. This shows that the property of elements of combining with such different elements as oxygen and hydrogen is subject to onecommon law, which is also formulated in the system of the elements presently to be described.[7]
In the preceding we see not only the regularity and simplicity which govern the formation and properties of the oxides and of all the compounds of the elements, but also a fresh and exact means for recognising the analogy of elements. Analogous elements give compounds of analogous types, both higher and lower. If CO2and SO2are two gases which closely resemble each other both in their physical and chemical properties, the reason of this must be looked for not in an analogy of sulphur and carbon, but in that identity of the type of combination, RX4, which both oxides assume, and in that influence which a large mass of oxygen always exerts on the properties of its compounds. In fact, there is little resemblance between carbon and sulphur, as is seen not only from the fact that CO2is thehigher formof oxidation, whilst SO2is able to further oxidise into SO3, but also from the fact that all the other compounds—for example, SH2and CH4, SCl2and CCl4, &c.—are entirely unlike both in type and in chemical properties. This absence of analogy in carbon and sulphur is especially clearly seen in the fact that the highest saline oxides are of different composition, CO2for carbon, and SO3for sulphur. InChapterVIII.we considered the limit to which carbon tends in its compounds, and in a similar manner there is for every element in its compounds a tendency to attain a certain highest limit RXn. This view was particularly developed in the middle of the present century by Frankland in studying the metallo-organic compounds,i.e.those in which X is wholly or partially a hydrocarbon radicle; for instance, X = CH3or C2H5&c. Thus, for example, antimony, Sb (ChapterXIX.) gives, with chlorine, compounds SbCl3and SbCl5and corresponding oxygen compounds Sb2O3and Sb2O5, whilst under the action of CH3I, C2H5I, or in general EI (where E is a hydrocarbon radicle of the paraffin series), upon antimony or its alloy with sodium there are formed SbE3(for example, Sb(CH3)3, boiling at about 81°), which, corresponding to the lower form of combination SbX3, are able to combine further with EI, or Cl2, or O, and to form compounds of the limiting type SbX5; for example, SbE4Cl corresponding to NH4Cl with the substitution of nitrogen by antimony, and of hydrogen by the hydrocarbon radicle. The elements which are most chemically analogous are characterised by the fact of their giving compounds of similar form RXn. The halogens which are analogous give both higher and lower compounds. So also do the metals of the alkalis and of the alkaline earths. And we saw that this analogy extends to the composition and properties of the nitrogen and hydrogen compounds of these metals, which is best seen in the salts. Many such groups of analogous elements have long been known. Thus there are analogues of oxygen, nitrogen, and carbon, and we shall meet with many such groups. But an acquaintance with them inevitably leads to the questions, what is the cause of analogy and what is the relation of one group to another? If these questions remain unanswered, it is easy to fall into error in the formation of the groups, because the notions of the degree of analogy will always be relative, and will not present any accuracy or distinctness Thus lithium is analogous in some respects to potassium and in others to magnesium; beryllium is analogous to both aluminium and magnesium. Thallium, as we shall afterwards see and as was observed on its discovery, has much kinship with lead and mercury, but some of its properties appertain to lithium and potassium. Naturally, where it is impossible to make measurements one is reluctantly obliged to limit oneself to approximate comparisons, founded on apparent signs which are not distinct and are wanting in exactitude. But in the elements there is one accurately measurable property, which is subject to no doubt—namely, that property which is expressed in their atomic weights. Its magnitude indicates the relative mass of the atom, or, if we avoid the conception of the atom, itsmagnitude shows the relation between the masses forming the chemical and independent individuals or elements. And according to the teaching of all exact data about the phenomena of nature, the mass of a substance is that property on which all its remaining properties must be dependent, because they are all determined by similar conditions or by those forces which act in the weight of a substance, and this is directly proportional to its mass. Therefore it is most natural to seek for a dependence between the properties and analogies of the elements on the one hand and their atomic weights on the other.
This is the fundamental idea which leadsto arranging all the elements according to their atomic weights. A periodic repetition of properties is then immediately observed in the elements. We are already familiar with examples of this:—
The essence of the matter is seen in these groups. The halogens have smaller atomic weights than the alkali metals, and the latter than the metals of the alkaline earths. Therefore,if all the elements be arranged in the order of their atomic weights, a periodic repetition of properties is obtained. This is expressed by thelaw of periodicity,the properties of the elements, as well as the forms and properties of their compounds, are in periodic dependence or (expressing ourselves algebraically) form a periodic function of the atomic weights of the elements.[8]Table I. ofthe periodic system of the elements, which isplaced at the very beginning of this book, is designed to illustrate this law. It is arranged in conformity with the eight types of oxides described in the preceding pages, and those elements which give the oxides, R2O and consequently salts RX, form the 1st group; the elements giving R2O2or RO as their highest grade of oxidation belong to the 2nd group; those giving R2O3as their highest oxides form the 3rd group, and so on; whilst the elements of all the groups which are nearest in their atomic weights are arranged in series from 1 to 12. The even and uneven series of the same groups present the same forms and limits, but differ in their properties, and therefore two contiguous series, one even and the other uneven—for instance, the 4th and 5th—form a period. Hence the elements of the 4th, 6th, 8th, 10th, and 12th, or of the 3rd, 5th, 7th, 9th, and 11th, series form analogues, like the halogens, the alkali metals, &c. The conjunction of two series, one evenand one contiguous uneven series, thus forms one largeperiod. These periods, beginning with the alkali metals, end with the halogens. The elements of the first two series have the lowest atomic weights, and in consequence of this very circumstance, although they bear the general properties of a group, they still show many peculiar and independent properties.[9]Thus fluorine, as we know, differs in many points from the other halogens, and lithium from the other alkali metals, and so on. These lightest elements may be termedtypical elements. They include—
H.
Li, Be, B, C, N, O, F.
Na, Mg....
In the annexed table all the remaining elements are arranged, not in groups and series, butaccording to periods. In order to understand the essence of the matter, it must be remembered that here the atomic weight gradually increases along a given line; for instance, in the line commencing with K = 39 and ending with Br = 80, the intermediate elements have intermediate atomic weights, as is clearly seen inTable III., where the elements stand in the order of their atomic weights.
The same degree of analogy that we know to exist between potassium, rubidium, and cæsium; or chlorine, bromine, and iodine; or calcium, strontium, and barium, also exists between the elements of the other vertical columns. Thus, for example, zinc, cadmium, and mercury, which are described in the following chapter, present a very close analogy with magnesium. For a true comprehension of the matter[10]itis very important to see that all the aspects of the distribution of the elements according to their atomic weights essentially express one and the same fundamentaldependence—periodic properties.[11]The following points then must be remarked in it.
1. The composition of the higher oxygen compounds is determined by the groups: the first group gives R2O, the second R2O2or RO, the third R2O3, &c. There are eight types of oxides and therefore eight groups. Two groups give a period, and the same type of oxide is met with twice in a period. For example, in the period beginning with potassium, oxides of the composition RO are formed by calcium and zinc, and of the composition RO3by molybdenum and tellurium. The oxides of the even series, of the same type, have stronger basic properties than the oxides of the uneven series, and the latter as a rule are endowed with an acid character. Therefore the elements which exclusively give bases, like the alkali metals, will be found at the commencement of the period, whilst such purely acid elements as the halogens will be at the end of the period. The interval will be occupied by intermediate elements, whose character and properties we shall afterwards describe. It must be observed that the acid character is chiefly proper to the elements with small atomic weights in the unevenseries, whilst the basic character is exhibited by the heavier elements in the even series. Hence elements which give acids chiefly predominate among the lightest (typical) elements, especially in the last groups; whilst the heaviest elements, even in the last groups (for instance, thallium, uranium) have a basic character. Thus the basic and acid characters of the higher oxides are determined (a) by the type of oxide, (b) by the even or uneven series, and (c) by the atomic weight.[11 bis]The groups are indicated by Roman numerals from I. to VIII.
2.The hydrogen compoundsbeing volatile or gaseous substances which are prone to reaction—such as HCl, H2O, H3N, and H4C[12]—are only formed by the elements of the uneven series and higher groups giving oxides of the forms R2On, RO3, R2O5, and RO2.
3. If an element gives a hydrogen compound, RXm, it forms anorgano-metallic compoundof the same composition, where X = CnH2n+1; that is, X is the radicle of a saturated hydrocarbon. The elements of the uneven series, which are incapable of giving hydrogen compounds, and give oxides of the forms RX, RX2, RX3, also give organo-metallic compounds of this form proper to the higher oxides. Thuszinc forms the oxide ZnO, salts ZnX2and zinc ethyl Zn(C2H5)2. The elements of the even series do not seem to form organo-metallic compounds at all; at least all efforts for their preparation have as yet been fruitless—for instance, in the case of titanium, zirconium, or iron.
4. The atomic weights of elements belonging to contiguous periods
differ approximately by 45; for example, K 5. According to the periodic system every element occupies a certain
position, determined by the group (indicated in Roman numerals)
and series (Arabic numerals) in which it occurs. These indicate the
atomic weight, the analogues, properties, and type of the higher oxide,
and of the hydrogen and other compounds—in a word, all the chief
quantitative and qualitative features of an element, although there yet
remain a whole series of further details and peculiarities whose causeshould perhaps be looked for in small differences of the atomic weights.
If in a certain group there occur elements, R1, R2, R3, and if in that
series which contains one of these elements, for instance R2, an
element Q2precedes it and an element T2succeeds it, then the properties
of R2are determined by the properties of R1, R3, Q2, and T2.
Thus, for instance, the atomic weight of R2= ¼(R1+ R3+ Q2+ T2).
For example, selenium occurs in the same group as sulphur, S = 32, and
tellurium, Te = 125, and, in the 7th series As = 75 stands before it and
Br = 80 after it. Hence the atomic weight of selenium should be
¼(32 + 125 + 75 + 80) = 78, which is near to the truth. Other properties
of selenium may also be determined in this manner. For example,
arsenic forms H3As, bromine gives HBr, and it is evident that selenium,
which stands between them, should form H2Se, with properties intermediate
between those of H3As and HBr. Even the physical
properties of selenium and its compounds, not to speak of their composition,
being determined by the group in which it occurs, may be foreseen
with a close approach to reality from the properties of sulphur, tellurium,
arsenic, and bromine.In this manner it is possible to foretell the properties
of still unknown elements.For instance in the position IV, 5—that
is, in the IVth group and 5th series—an element is still wanting.
These unknown elements may be named after the preceding known element
of the same group by adding to the first syllable the prefixeka-,
which meansonein Sanskrit. The element IV, 5, follows after IV, 3,
and this latter position being occupied by silicon, we call the unknown
element ekasilicon and its symbol Es. The following are
the properties which this element should have on the basis of the
known properties of silicon, tin, zinc, and arsenic. Its atomic weight
is nearly 72, higher oxide EsO2, lower oxide EsO, compounds of the
general form EsX4, and chemically unstable lower compounds of the
form EsX2. Es gives volatile organo-metallic compounds—for instance,
Es(CH3)4, Es(CH3)3Cl, and Es(C2H5)4, which boil at about 160°, &c.;
also a volatile and liquid chloride, EsCl4, boiling at about 90° and of
specific gravity about 1·9. EsO2will be the anhydride of a feeble colloidal
acid, metallic Es will be rather easily obtainable from the oxides
and from K2EsF6by reduction, EsS2will resemble SnS2and SiS2, and
will probably be soluble in ammonium sulphide; the specific gravity
of Es will be about 5·5, EsO2will have a density of about 4·7, &c.
Such a prediction of the properties of ekasilicon was made by me in
1871, on the basis of the properties of the elements analogous to it:
IV, 3, = Si, IV, 7 = Sn, and also II, 5 = Zn and V, 5 = As. And now
that this element has been discovered by C. Winkler, of Freiberg, it
has been found that its actual properties entirely correspond with thosewhich were foretold.[13]In this we see a most important confirmation
of the truth of the periodic law. This element is now called germanium,
Ge (seeChapterXVIII.). It is not the only one that has been
predicted by the periodic law.[14]We shall see in describing the elements
of the third group that properties were foretold of an element ekaaluminium,
III, 5, El = 68, and were afterwards verified when the
metal termed ‘gallium’ was discovered by De Boisbaudran. So also the
properties of scandium corresponded with those predicted for ekaboron,
according to Nilson.[15] 6. As a true law of nature is one to which there are no exceptions,
the periodic dependence of the properties on the atomic weights
of the elements gives anew means for determining by the equivalent
the atomic weightor atomicity of imperfectly investigated but
known elements, for which no other means could as yet be applied
for determining the true atomic weight. At the time (1869) when the
periodic law was first proposed there were several such elements. It
thus became possible to learn their true atomic weights, and these were
verified by later researches. Among the elements thus concerned were
indium, uranium, cerium, yttrium, and others. 7. The periodic variability of the properties of the elements in
dependence on their masses presents a distinction from other kinds
of periodic dependence (as, for example, the sines of angles vary
periodically and successively with the growth of the angles, or the
temperature of the atmosphere with the course of time), in that the
weights of the atoms do not increase gradually, but by leaps; that is,
according to Dalton's law of multiple proportions, there not only are
not, but there cannot be, any transitive or intermediate elements betweentwo neighbouring ones (for example, between K = 39 and Ca = 40, or
Al = 27 and Si = 28, or C = 12 and N = 14, &c.) As in a molecule
of a hydrogen compound there may be either one, as in HF, or two, as
in H2O, or three, as in NH3, &c., atoms of hydrogen; but as there cannot
be molecules containing 2½ atoms of hydrogen to one atom of another
element, so there cannot be any element intermediate between N and
O, with an atomic weight greater than 14 or less than 16, or between
K and Ca. Hence the periodic dependence of the elements cannot be
expressed by any algebraical continuous function in the same way that
it is possible, for instance, to express the variation of the temperature
during the course of a day or year. 8. The essence of the notions giving rise to the periodic law consists
in a general physico-mechanical principle which recognises the
correlation, transmutability, and equivalence of the forces of nature.
Gravitation, attraction at small distances, and many other phenomena
are in direct dependence on the mass of matter. It might therefore have
been expected that chemical forces would also depend on mass. A dependence
is in fact shown, the properties of elements and compounds
being determined by the masses of the atoms of which they are formed.
The weight of a molecule, or its mass, determines, as we have seen,
(ChapterVII.and elsewhere) many of its properties independently of
its composition. Thus carbonic oxide, CO, and nitrogen, N2, are two
gases having the same molecular weight, and many of their properties
(density, liquefaction, specific heat, &c.) are similar or nearly similar.
The differences dependent on the nature of a substance play another
part, and form magnitudes of another order. But the properties of
atoms are mainly determined by their mass or weight, and are in
dependence upon it. Only in this case there is a peculiarity in the
dependence of the properties on the mass, for thisdependence is determined
by a periodic law. As the mass increases the properties
vary, at first successively and regularly, and then return to their
original magnitude and recommence a fresh period of variation like
the first. Nevertheless here as in other cases a small variation of the
mass of the atom generally leads to a small variation of properties, and
determines differences of a second order. The atomic weights of cobalt
and nickel, of rhodium, ruthenium, and palladium, and of osmium,
iridium, and platinum, are very close to each other, and their properties
are also very much alike—the differences are not very perceptible.
And if the properties of atoms are a function of their weight,
many ideas which have more or less rooted themselves in chemistry
must suffer change and be developed and worked out in the sense of
this deduction. Although at first sight it appears that the chemicalelements are perfectly independent and individual, instead of this idea
of the nature of the elements, the notion of the dependence of their properties
upontheir massmust now be established; that is to say, the subjection
of the individuality of the elements to a common higher principle
which evinces itself in gravity and in all physico-chemical phenomena.
Many chemical deductions then acquire a new sense and significance,
and a regularity is observed where it would otherwise escape
attention. This is more particularly apparent in the physical properties,
to the consideration of which we shall afterwards turn, and we
will now point out that Gustavson first (Chapter X., Note28) and
subsequently Potilitzin (Chapter XI., Note66) demonstrated the direct
dependence of the reactive power on the atomic weight and that fundamental
property which is expressed in the forms of their compounds,
whilst in a number of other cases the purely chemical relations of elements
proved to be in connection with their periodic properties. As
a case in point, it may be mentioned that Carnelley remarked a dependence
of the decomposability of the hydrates on the position of the
elements in the periodic system; whilst L. Meyer, Willgerodt, and
others established a connection between the atomic weight or the
position of the elements in the periodic system and their property of
serving as media in the transference of the halogens to the hydrocarbons.[16]Bailey pointed out a periodicity in the stability (under the
action of heat) of the oxides, namely: (a) in the even series (for
instance, CrO3, MoO3, WO3, and UO3) the higher oxides of a given
group decompose with greater ease the smaller the atomic weight,
while in the uneven series (for example, CO2, GeO2, SnO2, and PbO2)
the contrary is the case; and (b) the stability of the higher saline
oxides in the even series (as in the fourth series from K2O to
Mn2O7) decreases in passing from the lower to the higher groups,
while in the uneven series it increases from the Ist to the IVth group,
and then falls from the IVth to the VIIth; for instance, in the seriesAg2O, CdO, In2O3, SnO2, and then SnO2, Sb2O5, TeO3, I2O7.
K. Winkler looked for and actually found (1890) a dependence between
the reducibility of the metals by magnesium and their position in the
periodic system of the elements. The greater the attention paid to
this field the more often is a distinct connection found between the
variation of purely chemical properties of analogous substances and
the variation of the atomic weights of the constituent elements and
their position in the periodic system. Besides, since the periodic
system has become more firmly established, many facts have been
gathered, showing that there are many similarities between Sn and
Pb, B and Al, Cd and Hg, &c., which had not been previously observed,
although foreseen in some cases, and a consequence of the periodic law.
Keeping our attention in the same direction, we see that the most
widely distributed elements in nature are those with small atomic
weights, whilst in organisms the lightest elements exclusively predominate
(hydrogen, carbon, nitrogen, oxygen), whose small mass facilitates
those transformations which are proper to organisms. Poluta
(of Kharkoff), C. C. Botkin, Blake, Brenton, and others even discovered
a correlation between the physiological action of salts and other reagents
on organisms and the positions occupied in the periodic system
by the metals contained in them.[17] As, from the necessity of the case, the physical properties must be
in dependence on the composition of a substance,i.e.on the quality
and quantity of the elements forming it, so for them also a dependence
on the atomic weight of the component elements must be
expected, and consequently also on their periodic distribution. We
shall meet with repeated proofs of this in the further exposition of
our treatise, and for the present will content ourselves with citing
the discovery by Carnelley in 1879 of the dependence of the magnetic
properties of the elements on the position occupied by them in the
periodic system. Carnelley showed that all the elements of theevenseries(beginning with lithium, potassium, rubidium, cæsium) belong
to the number of magnetic (paramagnetic) substances; for example,
according to Faraday and others,[17 bis]C, N, O, K, Ti, Cr, Mn, Fe, Co,
Ni, Ce, are magnetic; and the elements of theuneven series are
diamagnetic, H, Na, Si, P, S, Cl, Cu, Zn, As, Se, Br, Ag, Cd, Sn, Sb,
I, Au, Hg, Tl, Pb, Bi. Carnelley also showed that themelting-pointof elements varies
periodically, as is seen by the figures inTable III.(nineteenth column),[18]where all the most trustworthy data are collected, and predominance
is given to those having maximum and minimum values.[19] There is no doubt that many other physical properties will, when
further studied, also prove to be in periodic dependence on the atomic
weights,[19 bis]but at present only a few are known with any completeness,
and we will only refer to the one which is the most easily and
frequently determined—namely, thespecific gravityin a solid and
liquid state, the more especially as its connection with the chemical
properties and relations of substances is shown at every step. Thus,
for instance, of all the metals those of the alkalis, and of all the non-metals
the halogens, are the most energetic in their reactions, and they
have the lowest specific gravity among the adjacent elements, as is seen
inTable III., column 17. Such are sodium, potassium, rubidium,
cæsium among the metals, and chlorine, bromine, and iodine among the
non-metals; and as such less energetic metals as iridium, platinum,
and gold (and even charcoal or the diamond) have the highest specific
gravity among the elements near to them in atomic weight; therefore
the degree of the condensation of matter evidently influences the
course of the transformations proper to a substance, and furthermore
this dependence on the atomic weight, although very complex, is of a
clearly periodic character. In order to account for this to some extent,
it may be imagined that the lightest elements are porous, and, like a
sponge, are easily penetrated by other substances, whilst the heavier
elements are more compressed, and give way with difficulty to the
insertion of other elements. These relations are best understood when,
instead of the specific gravities referring to a unit of volume,[20]theatomic volumes of the elements—that is, the quotientA/dof the atomicweightAby the specific gravityd—are taken for comparison. As,
according to the entire sense of the atomic theory, the actual matter
of a substance does not fill up its whole cubical contents, but is surrounded
by a medium (ethereal, as is generally imagined), like the stars
and planets which travel in the space of the heavens and fill it, with
greater or less intervals, so the quotientA/donly expresses themeanvolume corresponding to the sphere of the atoms, and therefore [3root]A/dis the mean distance between the centres of the atoms. For compounds
whose molecules weighM, the mean magnitude of the atomic volume is
obtained by dividing the mean molecular volumeM/dby the number
of atomsnin the molecule.[21]The above relations may easily be
expressed from this point of view by comparing the atomic volumes.
Those comparatively light elements which easily and frequently enter
into reaction have the greatest atomic volumes: sodium 23, potassium
45, rubidium 57, cæsium 71, and the halogens about 27; whilst with
those elements which enter into reaction with difficulty, the mean atomic
volume is small; for carbon in the form of a diamond it is less than
4, as charcoal about 6, for nickel and cobalt less than 7, for iridium
and platinum about 9. The remaining elements having atomic weights
and properties intermediate between those elements mentioned above
have also intermediate atomic volumes. Thereforethe specific gravities
and specific volumes of solids and liquids stand in periodic dependence
on the atomic weights, as is seen inTable III., where bothA(the
atomic weight) andd(the specific gravity), andA/d(specific volumes
of the atoms) are given (column 18). Thus we find that in the large periods beginning with lithium,
sodium, potassium, rubidium, cæsium, and ending with fluorine, chlorine,
bromine, iodine, the extreme members (energetic elements) have a
small density and large volume, whilst the intermediate substances
gradually increase in density and decrease in volume—that is, as the
atomic weight increases the density rises and falls, again rises and falls,and so on. Furthermore, the energy decreases as the density rises, and
the greatest density is proper to the atomically heaviest and least
energetic elements; for example, Os, Ir, Pt, Au, U. In order to explain the relation between the volumes of the elements
and of their compounds, the densities (column S) and volumes
(column M/s) of some of the higher saline oxides arranged in the same
order as in the case of the elements are given on p.36. For convenience
of comparison the volumes of the oxides are all calculated
per two atoms of an element combined with oxygen. For example,
the density of Al2O3= 4·0, weight Al2O3= 102, volume Al2O3= 25·5.
Whence, knowing the volume of aluminium to be 11, it is at once seen
that in the formation of aluminium oxide, 22 volumes of it give 25·5
volumes of oxide. A distinct periodicity may also be observed with
respect to the specific gravities and volumes of the higher saline oxides.
Thus in each period, beginning with the alkali metals, the specific
gravity of the oxides first rises, reaches a maximum, and then falls on
passing to the acid oxides, and again becomes a minimum about the
halogens. But it is especially important to call attention to the fact
that the volume of the alkali oxides is less than that of the metal contained
in them, which is also expressed in the last column, giving this
difference for each atom of oxygen.[22]Thus 2 atoms of sodium, or
46 volumes, give 24 volumes of Na2O, and about 37 volumes of 2NaHO—that
is, the oxygen and hydrogen in distributing themselves in the
medium of sodium have not only not increased the distance between
its atoms, but have brought them nearer together, have drawn them
together by the force of their great affinity, by reason, it may be
presumed, of the small mutual attraction of the atoms of sodium.
Such metals as aluminium and zinc, in combining with oxygen and
forming oxides of feeble salt-forming capacity, hardly vary in volume,
but the common metals and non-metals, and especially those forming
acid oxides, always give an increased volume when oxidised—that is,
the atoms are set further apart in order to make room for the oxygen.
The oxygen in them does not compress the molecule as in the alkalis;
it is therefore comparatively easily disengaged. As the volumes of the chlorides, organo-metallic and all other
corresponding compounds, also vary in a like periodic succession with a
change of elements, it is evidently possible to indicate the properties
of substances yet uninvestigated by experimental means, and even those
of yet undiscovered elements. It was possible by following this method
to foretell, on the basis of the periodic law, many of the properties of
scandium, gallium, and germanium, which were verified with great
accuracy after these metals had been discovered.[23]The periodic law,
therefore, has not only embraced the mutual relations of the elements
and expressed their analogy, but has also to a certain extent subjected
to law the doctrine of the types of the compounds formed by the
elements: it has enabled us to see a regularity in the variation of
all chemical and physical properties of elements and compounds, and
has rendered it possible to foretell the properties of elements and
compounds yet uninvestigated by experimental means; thus it has
prepared the ground for the building up of atomic and molecular
mechanics.[24]
Back to IndexNext