372 Mr. J. P. Cooke on 



in gram units. Two series of reactions may now be arranged 

 so as to fulfil the conditions we have assumed : — 



First Series. 



K 6 + Br 6 (gas) + Aq= (6 KBr + Aq) . 570,000 units. 



A1 2 + C1 6 =A1 9 C1 6 321,800 „ 



Al 2 Cl 6 dissolved in (6 KBr + Aq) . . 152,000 „ 



1,043,800 



Second Series, 



K 6 + Cl 6 + Aq=(6KCl + Aq) .... 604,800 units. 



Al 2 + Br 6 =Al 2 Br 6 x 



Al 2 Br 6 dissolved in (6 KC1 + Aq) . . 173,800 



778,600 



?? 



x + 778,600 = 1 ,043,803. x= 265,200. 



In studying these two series of reactions, it will be evident 

 that we begin in each case with the same amounts of the same 

 elementary substances, namely K 6 , Al 2 , Br 6 , Cl 6 , and that we 

 end with aqueous solutions in the same condition. Hence the 

 total amount of heat evolved in each of the two series must 

 be the same, and we can at once deduce the value of the only 

 unknown quantity. In this determination, the only quantities 

 which had to be measueed at the time were the heat of solu- 

 tion of aluminic chloride in an aqueous solution of potassic 

 bromide on the one hand, and the heat of solution of aluminic 

 bromide in an aqueous solution of potassic chloride on the 

 other, using, of course, equivalent quantities in each case. 

 The other values given had previously been determined by 

 indirect methods ; and it can easily be seen that the investiga- 

 tion displays, not only a great command of knowledge, but 

 also a great fertility of invention ; and yet this is a compara- 

 tively simple case. 



As deductions from the general principle of the conservation 

 of energy, Berthelot gives a large number of theorems which 

 serve to illustrate the extent and variety of its application to 

 the study of the thermal changes which accompany chemical 

 reaction. We give two as examples : — 



Theorem III. — In two series of reactions starting from dif- 

 ferent initial conditions, but ending in the same final state, the 

 difference in the quantities of heat evolved is equal to that which 

 would be evolved in passing from one of the initial states to the 

 other. 



This theorem enables us to determine very simply the 



