128 HEAT. 



obtained by different experimenters are exhibited in terms of the varia- 

 tion of the specific heat of water. 



An inspection of these numbers shows that the results by the two 

 electrical methods are somewhat closely in agreement, and higher than 

 those given by water-friction methods. It appears probable that there 

 is some error in the electrical relations assumed, and some recent ex- 

 periments suggest that this is in the value of the E.M.F. of the Clark's 

 cell. Meanwhile, Rowland's corrected value at 15 may be taken, and 

 the first three figures of this may be put as 4'19 x 10 7 ergs. Hence we 

 have the mechanical equivalent 



4-1 9 x 10 7 ergs; 



4'27 metre grammes ; 



778 foot-lbs at Greenwich ; 



in terms of degrees on the hydrogen thermometer about 15* 0., the last, 

 of course, on the Fahrenheit scale. 



The First Law of Thermodynamics. When exchange occurs 



between work and heat, the researches we have been describing all 

 tend to show that the ratio of exchange is fixed. Hence if W is the 

 work in ergs equivalent to heat H measured in calories, we have 



W-JH 



where J is the mechanical equivalent, taken provisionally as equal to 

 4' 19 x 10 7 . This relation is known as the First Law of Thermo- 

 dynamics. 



