262 Prof. Apjohn on the Specific Heats of the GaseSf 



mine. Let me not, however, be misunderstood. Dr. Suer- 

 man has borrowed nothing from me, for before he had seen 

 my first paper on specific heats he had resolved upon em- 

 ploying the method in question in the same research*. So 

 far from having reclamations to make, I feel myself his debtor. 

 He has frankly admitted my priority, and spoken of my ex- 

 periments in terms I fear much too flattering. 



Before proceeding to my immediate object it will be neces- 

 sary to remind the reader that in November ISSif? I commu- 

 nicated the following formula for the solution of the dew-point 

 problem to the Royal Irish Academy : 



/-// = /^ _ A X -^ 

 ■^ -^ 88 30 ' 



in which y" is the force of vapour at the dew-point, f the 

 same at the temperature of the wet thermometer, d = f — t' 

 the difference between the indications of the wet and dry in- 

 struments, p the existing, and 30 the mean pressure. 



In investigating this expression it was assumed that the spe- 

 cific heat of air and the caloric of elasticity of aqueous vapour 

 are constant, at least within the limits of the variations, in these 

 latitudes, of atmospherical temperature and pressure, an hy- 

 pothesis the strict accuracy of which cannot be admitted. 

 Preparatory therefore to the application of the formula to the 

 investigation of the specific heats of gases, it became necessary 

 to give it its most comprehensive form, substituting for the 



numeral coefficient — — the factors of which it is composed, 



88 



and introducing the consideration of density, in order that 

 the expression may be true generally of the various elastic 

 fluids. The steps which have conducted to such general ex- 

 pression I shall here give, partly because I have not published 

 them elsewhere, and partly because my investigation of the 



d 7) 



formulay" r=i f — — — - x ~~ has been by some considered 



as complicated and obscure. When, in the case of the wet 

 thermometer, the stationary temperature is attained, the ca- 

 loric which vaporizes the water is necessarily exactly equal to 

 that which the surrounding gas imparts in descending from 

 its proper temperature to that of the moistened bulb. From 



» " Tandem opus aggressus, et occupatus in idoneo parando supellectili, 

 diarium accepi Anglicum, quo in collegio, quod Dublini habetur, Chemiae 

 professoris Apjohn continei)atur disquisitio, ex eodem illo principio 

 fluidorum elasticorum calorem specificum derivans. Primum, quid sileam? 

 Animo despondebam, quum novitatis colorem, quae mihi praecipue arride- 

 bat, de nieo evanescere viderem proposito." (Preface, p. viii.) 



[t See Lend, and Edinb. Phil. Mag. vol. vi. p. 182.] 



