386 Proceedings of Royal Society of Edinburgh, [july 18, 
The heat of solution appears to me to answer this question at once 
when we take into account that Sr acts upon 0 with less energy 
than Ba does. Thus — 
[Ba,Cl 2 ] — [Sr, Cl 2 ] =10190 [BaCl 2 ,Aq] 2070 
[Ba,0,Aq] - [Sr,0,Aq] 980 [SrCl 2 Aq] 11140 
Difference + 9210 
9070 
or, in other words, the heat deficient in the combination [Sr, Cl 2 ] as 
compared with [Ba,Cl 2 ] appears in the extra heat of solution of 
SrCl 2 as compared with BaCl 2 . This is not an isolated case, but 
appears in every chloride, so that if the heat of combination with O 
of the various metals was constant, the heat of solution would vary 
inversely in every case as the heat of combination. The slight 
difference of 140 units in above case appears as a difference in the 
heats of neutrality of the two salts. 
We can, however, get rid of all consideration of the heats of com- 
bination with oxygen by the help of the formula already given, 
( M"C1 2 \ _ ( M"Br 2 | 
t - H 2 ,Cl 2 ,Aqj ” { - H 2 ,Br 2 ,Aq j 
{M"Br 2 ,Aq} - {M"Cl 2 ,Aq} , 
for we can take the metals in pairs and have on the one side of the 
equation only the differences of the heats of combination, and on 
the other the differences of the heats of solution, and we shall see 
they are exactly complementary ; as the one increases the other 
decreases. That is to say, the more energy that is run down to the 
form of heat in the formation of any salt the less energy is there 
left to run down to the same form in solution, and vice versa. In 
fact, the one is entirely dependent on the other, and it seems to me 
absolutely certain that if the one phenomenon is due to chemical 
affinity, so is the other. The following list, taken at random, of 
bromides, chlorides, sulphates, and nitrates will show this : — 
Heat of Combination. Difference 
Heat of Solution. 
Difference. 
[Ba,Cl 2 ] 
- [Ba,Br 2 ] = 24780 
- 2910 
[Sr, Cl 2 ] 
- [Sr,Br 2 ] = 26850 
- 4970 
2070 
• + 
2060 
[Sr, Cl 2 ] 
— [Sr,Br 2 ] = 26850 
- 4970 
[Ca,Cl 2 ] 
- [Ca,Br 2 ] = 28970 
- 7100 
2120 
+ 
2120 
