136 On the Determination of the [Aug. 



entering gas will absorb a part of this excess of heat, and the 

 mixture will have a mean temperature between that of the enter- 

 ing gas and that which the air would have acquired if it had not 

 been obliged to part with any of its heat, Now it is evident that 

 this mean temperature will fie so much the lower the greater the 

 specific heat is of the gas which enters. The experiments made 

 by Mr. Leslie have led him to conclude, that equal volumes of 

 hydrogen and atmospherical air have the same specific heat. 



The principle upon which this ingenious process is founded is 

 not perfectly just ; since it appears from the experiments of 

 Gay-Lussac,* that part of the heat developed in this case comes 

 from the gas that enters into the receiver. It appears, likewise, 

 that some unknown circumstance has misled Mr. Leslie respect- 

 ing the result of his experiments : for analogous experiments, 

 made with the greatest care, have given different results to Gay- 

 Lussac; and he did not observe the equality of effect, which 

 takes place, according to Mr. Leslie, when atmospherical air 

 and hydrogen gas arc made 'to enter into an exhausted receiver. 

 Gay-Lussac, in these experiments, made use of two similar 

 balloons, communicating with each other by a pipe furnished 

 with a stop-cock. He made a vacuum in the one, and filled the 

 other successively with different dried gases. In the centre of 

 each of these balloons was a thermometer of spirit of wine. 

 When the stop-cock was turned, the gas rushed from the full 

 vessel into the empty one. He had taken precautions to render 

 the velocity of the current equal in all cases. The thermometer 

 of the first balloon sunk, and that of the second rose the same 

 number of degrees ; but that number varied according to the 

 nature and density of the gases employed. Gay-Lussac, think- 

 ing that the specific heat of the gases subjected to these experi- 

 ments was proportional to the rising and falling of the thermo- 

 meters, thought himself entitled to conclude that the specific 

 heat of equal volumes of the different gases was inversely as 

 their specific gravity, and of the same gas directly as its density; 

 but he only gave this opinion as a probable conjecture, without 

 affirming any thing respecting its justness. In fact, the pheno- 

 mena which take place in such cases are very complicated, and 

 it is almost impossible to distinguish what depends upon the 

 different conducting power of the gases from what depends upon 

 their specific heat. 



M. Gay-Lussac has discovered this himself, in the new 

 memoir which he has lately published on the subject, f and in 

 which he came to different results. He employed in these last 

 experiments a very simple and ingenious method to determine 

 the specific heat of the gases. It consists in passing to the centre 



t Mem. d'Arcueil, i. 180. + Aqn. de Clura, lxxxi. 98. 



