36 



HEAT. 



freezes again while filtrating through the 

 ice in the lower part of it ; a circum- 

 stance very unfavourable to the accu- 

 racy of the experiment. 



It occurred to Meyer, that the com- 

 parative quantities of caloric existing in 

 bodies may be ascertained, by marking 

 the times which equal quantities of them 

 require to cool a certain number of de- 

 grees, accounting their capacities to be 

 as these times, estimated by the volume ; 

 or, if divided by the specific gravity of 

 the substance, giving the capacity as 

 estimated by the weight. This plan has 

 been used by Leslie and Dalton, who 

 consider it less liable to objection than 

 any of the preceding, although it might 

 easily be shown that its accuracy is very 

 doubtful. 



Great difficulties are opposed to the 

 investigation of the comparative quan- 

 tities of caloric belonging to elastic 

 fluids at different temperatures. The 

 method by which this investigation has 

 been most frequently pursued, is to 

 watch the degrees of heat imparted, by 

 given portions of these elastic fluids, 

 cooling through a certain range of tem- 

 perature ; and, notwithstanding the 

 difficulties alluded to, Dr. Crawford is 

 believed to have made near approxima- 

 tions to truth, in his numerous delicate 

 and ingenious experiments. 



Mr. Leslie proposed to ascertain the 

 capacities of elastic fluids by the follow- 

 ing method : The capacities of elastic 

 fluids for heat being increased by ra- 

 refaction, he proposed to exhaust a 

 receiver, by the air-pump, of a portion 

 of its air ; the receiver having a delicate 

 thermometer suspended within it, and 

 the apparatus being allowed to acquire 

 the temperature of the room, more air 

 is admitted, which occasioning the con- 

 densation of the rarefied air, its capacity 

 is reduced, and heat consequently set 

 free. By repeating the experiment se- 

 veral times, with common air, and then 

 by comparing the effect produced upon 

 other gases, with that produced upon 

 common air, Mr. Leslie thinks that the 

 capacities of elastic bodies in general 

 might be ascertained. This plan has 

 been strongly objected to by M. Gay 

 Lussac, who believes it to be incapable 

 of accuracy. 



It appears from the experiments of 

 the last-named philosopher, that a part 

 of the heat in Mr. Leslie's process is 

 derived from the gas that is permitted 

 to enter into the receiver after the ra- 



refaction, by which the results obtained 

 would be materially affected. It also 

 appeared to M. Gay Lussac, that some 

 unknown circumstances must have 

 misled Mr. Leslie in judging of his ex- 

 periments, since similar experiments 

 made with great care by the French 

 philosopher gave different results. Mr. 

 Leslie inferred, from the experiments 

 alluded to, that the specific heats of hy- 

 drogen gas and common air, in equal 

 volumes, are the same. Gay Lussac, 

 in operating upon these elastic fluids 

 according to Mr. Leslie's method, ob- 

 served a considerable difference in the 

 effects produced upon them ; and, with- 

 out coming to a positive decision, 

 thought there is reason to conclude, 

 that the specific heat of equal volumes 

 of the different gases is inversely as 

 their specific gravity ; and of the same 

 gas directly as its density. 



For the purpose of determining the 

 specific heats of elastic fluids, Gay 

 Lussac contrived that a hot current of 

 one gas should meet a cold current of 

 another gas, in the centre of a small 

 reservoir containing a thermometer : 

 the temperature of the mixture was 

 then ascertained ; and knowing also the 

 temperature of each before they were 

 permitted to mix, it was easy to infer 

 the ratio between their respective spe- 

 cific heats. 



The capacities of numerous gaseous 

 bodies have been calculated by Mr. 

 Dalton upon a plan peculiar to himself, 

 and which is founded upon the supposi- 

 tion that the ultimate particles of all elas- 

 tic fluids contain the same quantity of 

 heat under the same pressure and tempe- 

 rature. The following TABLE gives his 

 results : 



Hydrogen gas 9 



Azotic gas 



Atmospheric air 



Ammonia 



defiant gas 



Oxygen 



Carburetted hydrogen 



Aqueous vapour 



Vapour of ether 



Nitrous gas 



Oxide of carbon 



Vapour of alcohol 



Sulphuretted hydrogen 



Nitrous oxide gas 



Vapour of nitric acid 



Carbonic acid 



Muriatic acid . 



382 



.759 

 .555 

 .555 

 .333 

 .333 

 .166 

 .848 

 .777 

 .777 

 .586 

 ,583 

 .549 

 .491 

 .491 

 ,424 



Berard and Delaroche caused a uni- 



