45 



had preceded the author in the same investigation, parti- 

 cularly those of Dulong. Dr. Apjohn's own method was 

 then detailed. In a paper read by him before the Academy 



in April, 1835, the equation*/" znf X^ was proved 



to include the solution of the dew-point problem. But the 

 factor a in this expression, which is obviously equal (when 

 the air or gas is dry, or in other words, when/"=i:0) to 



T-T-,X — , is the specific heat under a eiven volume of the 

 48t/ V 



gas which is supposed to be the subject of experiment. 



Hence \{ f and d be determined for the various aeriform 



fluids by observation, their relative capacities for caloric 



can be compared. Such is the principle of the method. 



Two distinct series of experiments were then detailed, 



from the second of which, as comprehending those which he 



conceives to be most accurate, the author has deduced the 



following table of specific heats : 



Specific Heats of equal Folumes. 



Atmospheric Air, 1.000 



Nitrogen, 1.048 



Oxygen, (by calculation,) . . .808 



Hydrogen, 1.459 



Carbonic Acid, 1.195 



Carbonic Oxide 996 



Nitrous Oxide, ..... 1.193. 



Dr. x\pjohn conceives himself justified in drawing from 

 his researches the following conclusions : 



1°. All gases have not under equal volumes the same 

 specific heat. 



2°. This law is not even true of the simple gases. 



* d = t — t' the difference of the temperatures shown by a wet and dry ther- 

 mometer, and/' ii the elastic force of vapour at temperature t'. 



