A system of chemistry, in four volumes (Volume 1).

  • Thomson, Thomas, 1773-1852.
Date:
1818
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Licence: Public Domain Mark

Credit: A system of chemistry, in four volumes (Volume 1). Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    £ DIVISION 1. they accounted for certain appearances. Of course, as soon as these appearances were explained without their use, every reason for supposing their existence was destroyed. The only fact which gives any countenance to the opinion that cold is a body, has been furnished by the following very curious experiment of Mr. Pictet.* Two concave tin mirrors being placed at the distance of 10| feet from each other, a very delicate air ther- mometer was put into one of the foci, and a glass matrass full of snow into the other. The thermometer sunk several degrees, and rose again when the matrass was removed. When nitric acid was poured upon the snow (which increases the cold), the thermometer sunk 5° or 6° lower. Here cold seems to have been emitted by the snow, and reflected by the mirrors to the thermometer, which could not happen unless cold were a substance. But this curious experiment is explained in a satisfactory manner, by applying to it Prevost's theory of radiant heat. We see from that theory that the fall of the thermometer is really owing to a smaller proportion than usual of heat being radiated. A very great degree of cold may be produced by mixing together different solids, which suddenly become liquid. The cause of this has been already explained. But as such mixtures are often em- ployed in chemistry, in order to be able to expose bodies to the in- fluence of a low temperature, it will be worth while to enumerate the different substances which may be employed for that purpose, and the degree of cold which each of them is capable of producing. The first person who made experiments on freezing mixtures was Fahrenheit. But the subject was much more completely investi- gated by Mr. Walker in various papers published in the Philoso- phical Transactions from 1787 to 1801. Several curious additions have been made by Professor Lowitz, particularly the introduction of muriate of lime, which produces a very great degree of cold when mixed with snow.f The experiments of Lowitz have been repeated and extended by Mr. Walker4 The result of all these experiments may be seen in the following tables, which I transcribe from a paper with which I have been favoured by Mr. Walker. * This experiment, or at least a similar one, was made long ago, and is found in the Es- says ot the Academy del Cimento, translated by Waller in 1684, p. 103 The ninth expe- riment, ot reflected cold, is thus related : « We were willing to try, if a concave glass, set be- fore a mass of 5001bs. ot ice, made any sensible repercussion of cold upon a very nice ther- mometer of 400 degrees, placed in its focus. The truth is, it immediately began to subside; but, by reason of the nearness of the ice, it was doubtful whether the direct or reflected rays of cold were more efficacious: upon this account, we thought of covering the irlass and (whatever may be the cause) the spirit of wine did indeed presently begin to rise: for all this, we dare not be positive hut there might be some other cause thereof besides the ■want of the reflection from the glass, since we were deficient in making Hll the trials neces- sary to clear the experiment/' The experiment of Pictet appears likewise to have been ™ «dee °y Ad'ew Gaertner, Mechanician to the King of Poland, and published by him in 1785. See Ann. de Chim. Ixxi. 159.—[Si quis candelam in loco ubi spectabilis res locari debet, opposuent,accedat candela per aerem usque ad oculos ut illoscalore et lumine often- det; hoc autem mirabilius erit, ut calor, ita fngus refiectitur, « eo loco, tux obiiciatur. si oculnm retigent, qvia sensibilis etiamfrigus percibiet. Sed admirabilior est quod[idem spe- culum non solum calorem et frigus, sed vocem refringit. Baptista Porta mae-ia Naturalis ■ editio septima xvi. 4. 26 Nicholson's Journal, 314.—C] g naturaus. t Ann.de Chim. xxii. 297. and xxix. 281. + Phil. Trans. 1801, p. 120.