CHEMISTRY. 



37 



Blcmenu of water is HO 8 . The following Table exhibits the 

 f latent heat of some other liquids, as ascertained by 

 Chemistry. tne experiments of Dr Irvine : 



Kvapora- 



Boiling. 



Dr Black has shewn also, that the softness of such 

 bodies as are rendered plastic by heat! depends up- 

 on their combining with a quantity of caloric. 



2. Thus the conversion of solids into liquids is 

 owing to their combining with heat. There is an- 

 other change no less remarkable, to which bodies are 

 liable when exposed to the action of heat. Almost 

 all liquids, when exposed to a certain temperature, 

 gradually assume the form of an elastic fluid, pos- 

 sessing the properties of air. These fluids retain 

 their clastic form as long as the temperature conti- 

 nues, but when cooled down they lose that form, and 

 are converted into liquids. 



Some liquids are gradually converted into elastic 

 fluids at all temperatures, while others do not begin 

 to undergo the change till heated to a certain tempe- 

 rature. Water and alcohol are well known examples 

 of the first class of liquids ; sulphuric acid, and the 

 fixed oils, of the second. Water, gradually evapo- 

 rates, even when in the state of ice, but sulphuric 

 acid not till heated above 212. The first class of 

 liquids are said to evaporate spontaneously. 



When other circumstances are the same, the eva- 

 poration increases with the temperature, and the elas- 

 ticity of the vapour, of course, increases in the same 

 proportion. At a certain temperature this elasticity 

 balances the pressure of the atmosphere. When that 

 happens, if the heat be applied below, the liquid as- 

 sumes the aerial form with great rapidity. The va- 

 pour forces its way through the liquid, and a violent 

 agitation is the consequence. The liquid is then said 

 to boil. Every particular liquid has a certain tempe- 

 rature at which it begins to boil. Thus, ether boils 

 at 98, alcohol at 174, and water at 2 1 '2". 



The boiling point varies with the pressure of the 

 atmosphere. It is highest when the barometer is 

 high, and lowest when it is low. All liquids boil in 

 a vacuum about 145 lower than under the pressure 

 of the atmosphere. The elasticity of vapour increa- 

 ses with the temperature. At 32 a the vapour of wa- 

 ter is capable of supporting a column of mercury 0.2 

 inches high ; at 212 it supports a column of 30 

 inche*. 



Dr Black applied his theory of latent heat to the 

 conversion of liquids into elastic fluids, and bhowcd 

 that it is owing to the very same cause as the con- 

 version of solids into liquids, namely, to the combi- 



nation of a certain dose of caloric with the liquid Elements 

 without any increase of temperature. From his ex- f 

 periments, compared with those of Mr Watt and Mr ._ e !' a _j 

 Lavoisier, it appears that the latent heat of steam is ^~" 1 

 about 1000. 



Thus it appears that Dr Black's law is very ge- 

 neral, and comprehends every change in the state of 

 a body. It may be stated in its most general form 

 as follows. Whenever a body changes its state, it ei- 

 ther combines with caloric, or separates from caloric. 



3. It is probable, that all elastic fluids, or gases, 

 owe their elastic form, like steam, to the combined 

 caloric which they contain ; and that, if they could 

 be subjected to a sufficient degree of cold, they would 

 lose their elasticity, and be converted into liquids or 

 solids. This has been done successfully to some 

 gases ; oxymuriatic acid and ammonia, for example, 

 become liquid, when cooled down low enough. The 

 experiment has not succeeded with other gases, even 

 though subjected at once to cold and pressure. 



3. Changes in Composition, 



Caloric not only increases the bulk of bodies and Changes in 

 changes their state, but its action decomposes many composi- 

 compounds altogether, either into their elements, or tlon ' 

 it causes these elements to combine in a different man- 

 ner. Thus, ammonia, in a red heat, is resolved into 

 hydrogen and azotic gases ; and alcohol, by the same 

 heat, is converted into inflammable air and water. 



In general, those compounds, which have been 

 formed by combustion, resist the action of heat with 

 considerable obstinacy. Those that contain oxygen, 

 and which have been formed without combustion, are 

 easily decomposed, and so are most of those that con- 

 tain combustibles. 



SECT. V. Of the Quantity of Caloric in Bodies. 



This investigation naturally divides itself into two Of the 

 parts : 1 . The relative quantities of heat in bodies, quantity of 

 or the quantities in each necessary to produce a given P'?. r ' c * n 

 change of temperature. This is usually termed spe- **' 

 cific caloric. 2. The absolute quantity of heat which 

 exists in bodies. 



1. Of the Specific Caloric of Bodies. 



If equal weighte of water and spermaceti oil be 

 mixed at different temperatures, it is natural to ex- 

 pect that the mixture will acquire the mean tempera- 

 ture. Suppose the temperature of the water 100, Specific 

 and that of the oil 50, it is reasonable to expect that "?*.' e *~ 

 the water would be cooled down 25, and that the p 

 oil would be heated 25, and that the tempera- 

 ture after mixture would be 75. But, if. we 

 make the experiment, we find the result very diffe- 

 rent. The temperature, after mixture, instead of 75, 

 is 83y, consequently the water has. lost only 16j, 

 while the oil has gained 33^. If we mix. together 

 equal weights of water at 50, and spermaceti oil at 

 100, the temperature> after agitation, will be only 

 66j, oo that the oil has lost 33', while the water 

 has only gained Hi}-- This, experiment demonstrates 

 that the same quantity of heat does not produce the 

 same effect on water and spermaceti oil. The quan- 

 tity which raises water 16}, raises the oil 33?, or it 

 produces double the effect upon the oil that it does 



