SECTION VI.OF THE THERMOMETER.
Thisinstrument is designed for measuring the increase or decrease of heat. By doing it numerically, it fixes in our minds the quantity of fire, which any subject, at any time, is impregnated with. If different bodies are brought together, though each possesses a different degree of heat, it teaches us to discover what degree of heat they will arrive at when thoroughly mixed, supposing effervescence to produce no alteration in the mixture.
The inventor of this admirable instrument is not certainly known, though the merit of the discovery has been ascribed to several great men, of different nations, in order to do them and their countries honor. It came to us from Italy, about the beginning of the sixteenth century. The first inventors were far from bringing this instrument to its present degree of perfection. As it was not then hermetically sealed, the contained fluid was, at the same time, influenced by the weight of the air, and by the expansion of heat. The academy of Florence added this improvement to their thermometers, which soon made them more generally received; but, as the highest degree of heat of the instrument, constructed by the Florentine gentlemen, was fixed by the action of the strongest rays of the sun in their country, this vague determination, varying in almost every place, and the want of a fixed universal scale, rendered all the observations made with such thermometers of little use to us.
Boyle, Halley, Newton, and several other great men, thought this instrument highly worthy of their attention. They endeavoured to fix two invariable points to reckon from, and, by means of these, to establish a proper division. Monsieur des Amontons is said to have first made use of the degree of boiling water, for graduating his mercurial thermometers. Fahrenheit, indeed, found the pressure of the air, in its greatest latitude, would cause a variation of six degrees in that point; he therefore concluded, a thermometer made at the time when the air is in its middle state, might be sufficiently exact for almost every purpose. Long before the heat of boiling water was settled as a permanent degree, many means were proposed to determine another. The degree of temperature in a deep cave or cellar, where no external air could reach, was imagined by many a proper one; but what that degree truly was, and whether it was fixed and universal, was found too difficult to be determined. At last the freezing point of water was thought of, and though some doubts arose, with Dr. Halley and others, whether water constantly froze at the same degree ofcold, Dr. Martine has since, by several experiments, proved this to be beyond all doubt, and this degree is now received for as fixed a point as that of boiling water.
These two degrees being thus determined, the next business was the division of the intermediate space on some scale, that could be generally received. Though there seemed to be no difficulty in this, philosophers of different countries have not been uniform in their determinations, and that which is used in the thermometer at present the most common, and, in other respects, the most perfect, is far from being the simplest.
The liquid wherewith thermometers were to be filled, became the object of another enquiry. Sir Isaac Newton employed, for this purpose, linseed oil; but this, being an unctuous body, is apt to adhere to the sides of the glass, and, when suddenly affected by cold, for want of the parts which thus stick to the sides, does not shew the true degree.
Tinged water was employed by others; but this freezing, when Fahrenheit’s thermometer points 32 degrees, and boiling, when it rises to 212, was, from thence, incapable of denoting any more intense cold or heat.
Spirit of wine, which endures much cold without stagnating, was next made use of; but this liquor, being susceptible of no greater degree of heat than thatwhich, in Fahrenheit’s scale, is expressed by 175, could be of no service where boiling water was concerned.
At last the properest fluid, to answer every purpose, was found to be mercury. This had never been known to freeze6; and not to boil under a heat of 600 degrees, and is free from every inconveniency attending other liquors.
As the instrument is entirely founded on this principle, that heat or fire expands all bodies, as cold condenses them, there was a necessity of employing a fluid easy to be dilated. A quantity of it is seated in one part in the bulb. This being expanded by heat, is pushed forward into a fine tube, or capillary cylinder, so small, that the motion of the fluid in it is speedy and perceptible. Some thermometers have been constructed with their reservoir composed of a larger cylinder; but in general, at present, they are made globular. The smaller the bulb is, the sooner it is heated through, and the finer the tube, the greater will be the length of it, and the more distinct the degrees. It is scarcely possible that any glass cylinder, so very small, should be perfectly regular; the quicksilver, during the expansion, passing through some parts of the tube wider than others, thedegrees will be shorter in the first case, and longer in the latter. If the divisions, therefore, are made equal between the boiling and freezing points, a thermometer, whose cylinder is irregular, cannot be true. To rectify this inconveniency, the ingenious Mr. Bird, of London, puts into the tube about the length of an inch of mercury; and measuring, with a pair of compasses, the true extent of this body of quicksilver in one place, he moves it from one end to the other, carefully observing where it increases or diminishes in length, thereby ascertaining the parts, and how much the degrees are to be varied. By this contrivance, his thermometers are perfectly accurate, and exceed all that were ever made before.
I shall not trouble my reader with numerous calculations that have been made, to express the quantity of particles of the liquor contained in the bulb, in order to determine how much it is dilated. This, Dr. Martine seems to think a more curious than useful enquiry. It is sufficient, for our purpose, to know how the best thermometers ought to be constructed: they who have leisure and inclination, may be agreeably entertained by the author last cited.
By observing the rise of the mercury in the thermometer, during any given time, as, for instance, during the time of the day, we ascertain the degree and value of the heat of every part of the day, from whence maybe fixed the medium of the whole time, or any part thereof. By repeated experiments, it appears, the medium heat of most days is usually indicated at eight o’clock in the morning, if the instrument is placed in the shade, in a northern situation, and out of the reach of any accidental heat.
Though water is not so readily affected as air by heat and cold, yet, as all bodies long exposed in the same place, become of the same degree of heat with the air itself, no great error can arise from estimating water, in general, to be of the same heat as the air, at eight o’clock in the morning, in the shade.
The thermometer teaches us that the heat of boiling water is equal to 212 degrees, and by calculation we may know what quantity of cold water is necessary to bring it to any degree we choose; so, notwithstanding the instrument cannot be used in large vessels, where the water is heating, yet, by the power of numbers, the heat may be ascertained with the greatest accuracy. The rule is this: multiply 212, the heat of boiling water, by the number of barrels of water thus heated, (suppose 22) and the number of barrels of cold water to be added to the former, (suppose 10,) by the heat of the air at eight o’clock, (suppose 50,) add these two products together, and divide by the sum of the barrels; the quotient shews the degree of heat of the water mixed together.
(161⅓ degrees will be the heat of the water when mixed together.
The calculation may be extended to three or more bodies, provided they be brought to the same denomination. Suppose 32 barrels of water to be used where there is a grist of 20 quarters of malt, if these 20 quarters of malt are of a volume or bulk equal to 11 barrels of water, and the malt, by having lain exposed to the air, is of the same degree of heat with the air, in order to know the heat of the mash, the calculation must be thus continued.
We shall meet hereafter with some incidents, which occasion a difference in the calculations made for the purpose of brewing, but of these particular mention will be made in the practical part.
The thermometer, by shewing the different degrees of heat of each part of the year, informs us, at the same time, how necessary it is the proportions of boiling water to cold should be varied to effect an uniform intent; also that the heat of the extracts of small beer should differ proportionably as the heats of the seasons do: it assists usto fix the quantity of hops necessary to be used at different times; how much yeast is requisite, in each term of the year, to carry on a due fermentation; and what variation is to be made in the length of time that worts ought to boil. Indeed, without this knowledge, beers, though brewed in their due season, cannot be regularly fermented, and whenever they prove good, so often may it be said fortune was on the brewer’s side.
Beers are deposited in cellars, to prevent their being affected by the variations of heat and cold in the external air. By means of the thermometer, may be determined the heat of these cellars, the temper the liquor is kept in, and whether it will sooner or later come forward.
The brewing season, and the reason why such season is fittest for brewing, can only be discovered by this instrument. It points out likewise our chance for success, when necessity obliges us to brew in the summer months.
As all vegetable fermentation is carried on in heats, between two settled points, we are, by this instrument, taught to put our worts together at such a temperature, as they shall neither be evaporated by too great a heat, nor retarded by too much cold.
If curiosity should lead us so far, we might likewise determine, by it, the particular strength of each wort, or of every mash; for if water boils at 212 degrees, oil at 600, and worts be a composition of water, oil and salt;the more the heat of a boiling wort exceeds that of boiling water, the more oils and salts must it contain, or the stronger is the wort.
A given quantity of hops, boiled in a given quantity of water, must have a similar effect, consequently the intrinsic value of this vegetable may, in the same manner, be ascertained.
The more the malts are dried, the more do they alter in color, from a white to a light yellow, next to an amber, farther on to a brown, until the color becomes speckled with black; in which state we frequently see it. If more fire or heat is continued, the grain will at last charr, and become intirely black. By observing the degrees of heat necessary to induce these alterations, we may, by the mere inspection of the malt, know with what degree of fire it has been dried; and fixing upon such which best suits our purpose, direct, with the greatest accuracy, not only the heat of the first mash, but the mean heat the whole brewing should be impressed with to answer our intent, circumstances of the greatest consequence to the right management of the process.
If I had not already said enough to convince the brewer of the utility of this instrument, how curious he ought to be in the choice, and how well acquainted with the use of it, I should add the heat gained by the effervescing of malt, is to be determined by it alone; the quantity of heat lost by mashing, by the water in its passage from the copper to the mash ton, and by the extract coming down into the underback, these can be found by no other method; and, above all, that there is no other means to know with certainty the heat of every extract.
I know very well good beers were sometimes, perhaps often, made before the thermometer was known, and still is, by many who are entirely ignorant of it; but this, if not wholly the effect of chance, cannot be said to be very distant from it. They who carry on this process, unassisted by principles and the use of the thermometer, must admit they are frequently unsuccessful, whereas did they carefully and with knowledge apply this instrument, they certainly would not be disappointed.—It is equally true, the brewing art, for a long space of time, has been governed by an ill-conveyed tradition alone; if lucky combinations have sometimes flattered the best practitioners, faulty drinks have as often made them feel the want of certain and well established rules. It is just as absurd for a brewer to refuse the use of the thermometer, as it would be for an architect to reject the informations of his plummet and rule, and to assert they were unserviceable because the first house, and probably many others, were built without their assistance.