Calometric Absolute Measurements. 135 



As the general result of these 36 tolerably accordant expe- 

 riments (the extremes differing at the most only -J per cent, 

 from the mean) we get : — The mechanical equivalent of the 

 heat-unit, derived from the heat-evolution of the stationary 

 galvanic current, has the value 42815 metre-kilograms (with 

 the probable error ±0*22), understanding by unit of heat that 

 amount which must be supplied to the unit of mass (1 kilo- 

 gram) of water in order to raise its temperature 1° C. as mea- 

 sured by the air thermometer. 



The surest means for deriving, in a purely thermic way, the 

 quantitative value of the mechanical equivalent of the heat- 

 unit is unquestionably furnished by the relation between the 

 two specific heats of an ideal permanent gas — 



J(^-0=Wo a ; 

 or 



k — 1 

 JCp ~k~ = ^ a - 



For atmospheric air the three quantities p v , a, and c p are very 

 accurately known from Regnault's measurements : p v = 7991; 

 a = 0*00367 ; and c p = 0*23754. The quantity h has been more 

 recently determined for the same gas very carefully by M. 

 Rontgen: k= 1*4053. Inserting these numerical values in 

 the last equation, and also taking into account that, according 

 to the experiments of Joule and Thomson, atmospheric air ac- 

 complishes in alterations of volume, besides the external work 

 performed, an internal work equal to about -gfa of the external, 

 we obtain from the thermal behaviour of air 428*95 metre- 

 kilograms as the mechanical equivalent of the unit of heat. 

 The unit on which this number is based is that quantity of heat 

 which must be supplied to the mass-unit (1 kilogram) of water 

 at 14° or 15° in order to bring about a rise of temperature of 

 1° (measured by the air thermometer). 



Dr. Joule, in 1849, noted as the most trustworthy result of 

 his numerous experiments on friction for the determination of 

 the mechanical equivalent of the unit of heat the value 

 J = 423*79 metre-kilograms. In the calculation of this num- 

 ber the specific heat of water was put = 1 for the temperature 

 14°*4 ; moreover the specific heat of the calorimeter-vessel 

 was assumed too high. If the necessary correction on account 

 of the latter circumstance be added, the result just mentioned 

 becomes J = 424*39 metre-kilograms. The sixty friction- 

 experiments made quite recently by Joule have given almost 

 exactly the same result, 424*67 metre-kilograms. 



Unfortunately the total result of Joule's friction-experiments, 



