J. P. Cooke, Jr.—Berthelot's Thermo-Chemastry. 267 
water when the difference in the heat evolved in the two cases 
will be the quantity required. So, also, the heat of forma- 
in thermo-chemistry are said to be endothermous, while by far 
the larger class of compounds whose formation is attended 
with an evolution of heat are said to be exothermous. 
TuErorEM VI.— When a compound gives up one of its elements 
to another body, the heat evolved in the reaction is the difference 
between the heat of formation of the first compound and that of the 
resulting product. 
Thus, when an aqueous solution of chlorine is used as an 
oxidizing agent, for every eighteen grams of water decomposed 
9,600 units of heat are evolved, and this amount Is the differ- 
ence between the heat of formation of H,O and 2HCl. As can 
easily be seen, the same theroem applies to the problem pre- 
sented by explosive agents of various kinds and simplifies the 
solution to a remarkable extent. 
hese few illustrations will serve to give a general idea of the 
mode of investigation in this new field of thermo-chemuistry, 
but they are wholly inadequate to show either the extent o 
the field or the great skill with which it has been cultivated. 
em 
interesting relations which, under his third fundamental prin- 
sl Berthelot discusses in the second volume of his great 
wor 
[To be continued.] 
