346 Prof. Apjohn on the Specific Heats of the Gases, 



comparative view of our results which he gives at p. 89. 

 There is in fact a material difference between his numbers 

 and those which he ascribes to me ; for to confine ourselves 

 to a single example, the specific heat of hydrogen compared 

 to that of an equal volume of air as determined by him is 

 1*3979, and, by me (as he alleges) 1-8948. But if the 

 reader will refer to my first series of experiments, which alone 

 were seen by Suerman previous to the publication of his 

 Thesis, he will find the result for hydrogen to be 1*506, or 

 but very little higher than that arrived at by the Dutch phi- 

 losopher. The number therefore 1*8948 is not at all a con- 

 sequence of my experiments ; but it was nevertheless, as has 

 been already fully explained, very natural that it should have 

 been so considered by those who had merely seen the results 

 of my researches. 



The full explanation of this point has been already given, 

 so that it is sufficient for me to observe here that Suerman 

 was ignorant of it, and considering the numbers first published 

 by me as correctly representing my experiments, was not en- 

 abled to see that the general correspondence which he recog- 

 nised between the results of our investigations amounted to 

 an almost perfect identity. 



Having set myself right with the reader on these points, 

 I have next to draw attention to some observations of Suer- 

 man's of great interest and importance in relation to the prin- 

 ciple of the process which both he and I have adopted in in- 

 vestigating the question of the specific heats of the gases. In 

 some preliminary experiments performed by him, in which 

 a comparatively feeble pressure was employed, the experi- 

 mental depressions were much less than those which his for- 

 mula (that of Gay-Lussac) led him to expect. He therefore 

 augmented the pressure, with the view of augmenting the ra- 

 pidity of the current, and found that the experimental value 

 o^ t — t' was thus increased. What is the cause of this? and 

 is there any reason to believe that it is in our power to give 

 such a degree of velocity to the current as to conduct in every 

 instance not only to the maximum difference between the in- 

 dications of the wet and dry thermometers, but to such dif- 

 ference as may be considered to be the accurate measure of 

 the specific heat or quantity which it is sought to investigate ? 



The cause of the superior influence of the augmented cur- 

 rent is correctly assigned by Suerman. The caloric of ela- 

 sticity of the vapour derived from the water of the wet thermo- 

 meter comes chiefly from the air which impinges upon the 

 moistened bulb, but partly also by radiation from the sides of 

 the tube in which the instruments are placed. Now the caloric 



