The Theory and Practice of Brewing

by Michael Combrune

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 that which, 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 freeze[6]; 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, the degrees 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 may be 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.