192 Dr. Apjohn in reply to Dr. Hudson. 



to engage in this investigation at some early opportunity ; but 

 of this circumstance my remembrance is not so distinct as of 

 the other. BeUeve me always yours sincerely, 

 " Trinity College. June 13, 183G. " H. Lloyd." 



" Dr. Apjohn^ 



Having disposed of Dr. Hudson's claims of priority in rela- 

 tion to the suggestion of the hygrometrical experiments per- 

 formed by me, I shall nowr briefly advert to another topic. 



Almost immediately after having fallen on my hygrometric 

 formula 1 resolved, should I succeed in verifying it by experi- 

 ment, to apply it to the determination of the specific heats 

 of the gaseous bodies. That it may be employed in such re- 

 search is obvious, as the specific i)eat of the gas is a factor in the 

 formula. In fact, my expression in its most extended form, so 



. Ill- • r,t ,f 'iSs ad p 

 as to mclude the various gases, isj"=j — x ^> 



5 being the specific gravity and a the specific heat of the gas, 

 and e the caloric of elasticity of aqueous vapour whose elastic 

 force is y. From this it is easy to deduce a = {f'—f") 



X r X — » an equation which, when_/'' = 0, or the gas 



e 30 



is dry, becomes a = f x -rr, — i x — • If then an observa- 

 •" "^ 4^8 s d p 



tiou with a wet and dry thermometer be made in a number of 

 gases, their specific heats, that is, the value of a for each, may 

 be calculated. Such is the principle of my method. 



Upon this plan I have performed a number of experiments, 

 and am, I may observe, still occupied with the subject. Having, 

 however, on the Saturday previous to the meeting of the 

 British Association in this city, arrived, with the aid of a more 

 perfect apparatus than I had previously employed, at son)e, 

 as far as 1 could judge, very satisfactory results, I set myself 

 down, and in a very hurried manner submitted them to cal- 

 culation, and gave to the Chemical Section not so much a 

 formal paper on the subject as an account, chiefly verbal, of 

 my method, and of the results to which it conducted, resolving 

 of course to investigate the subject much more carefully, antl 

 to calculate much more at my leisure before I submitted my 

 investigations, as I always contemplated doing, to the Royal 

 Irish Academy. The ccmclusion really deducible from my 

 experiments was, to a certain extent, a confirmation of the 

 opinion which has of late been so much advocated, that all 

 gases have under equal volumes the same capacity for caloric*. 



• This idea, first started by Dalton, and since supported by the experi- 



