500 Scientific Proceedings, Royal Dublin Society. 
from the state “a” to the state “bd” will be that obtained by 
subtracting the first quantity from the second which we expres In 
our notation as :— 
E'ays) + E(eya) = Ey) 
E (eye) = E (rys) 3 E (ays). 
If in a reaction we desire the thermal value after any change 
of state in either the reagents or products, all we need do is to add 
to each the thermal values of the particular changes of state, be 
these values positive or negative, and retain the sign proper to the 
position of each in the equation, or we might put the rule thus, 
add to every positive quantity and subtract from every negative 
quantity the thermal value of the particular change be it positive 
or negative. or example, if gaseous hydrochloric acid combines 
with gaseous ammonia to form solid chloride of ammonia we have 
the equation 
£(NH, + HCl = NH,Cl] = 42:5K°. 
Find the thermal value of the reaction with the reagents and 
product in solution, given the heat of solution of ammonia, hydro- 
chloric acid, and ammonium chloride, respectively, as :— 
&(NH,) = + 8°435Ke . €(HCl) = 17-314K° . €(NH.Cl) =- 400K. 
é[NH, + HCl -=NH,Cl] = 42-5K°. 
— éNH; — cHCl + ENH,Cl = 42°5K°. 
— €°(NH,)- €\(HCl)+ €°(NH,C]) = —- — 8-485 K°- 17-314 K°- 4-00K°. 
—E€NH, -—€HCl +€NH,Cl =42°5-8:-485 -17:314 —4-00. 
é[NH, +HCl -=NH,Cl] = 12-751K°. 
When the reagents and products are in their normal state at 
15° C., or in dilute solution, there is as a rule no need to make 
mention of this, or it can be conveniently mentioned in the 
opening statement, and we may write at once our reagents with 
negative signs, our products with positive signs, and the evolved 
energy of the reaction in kilogram degrees, and then combine with 
