32 
DR. ANDREWS ON THE THERMAL CHANGES 
quantity of potash required for neutralization, the usual heat due to the substitution 
of potash for water will be obtained. 
I have formerly shown that there is a definite evolution of heat when solutions of 
the common alkaline phosphates and arseniates are mixed with a solution containing 
an additional equivalent of base, while no change of temperature occurs when a 
solution of the pyrophosphate of soda is similarly treated. In the former case, it has 
been shown by Mr. Graham that an atom of basic water is displaced by an atom of 
alkali ; in the latter case, no basic water is present. 
In the preceding observations it has been assumed, that if the union of two sub- 
stances be attended with the evolution of a certain definite quantity of heat, their 
separation will be attended with the absorption of the same quantity of heat. 
Although this proposition in the abstract is very probable, it requires to be demon- 
strated by direct experiment, and it is the more important to do so, as it will furnish, 
if true, a means of verifying the accuracy of our results. The reactions now described 
enable us to test it by experiment in one particular set of cases. In fact, if we take 
three bases, such as potash, oxide of copper, and water, capable of displacing one 
another in the above order, -and if we measure the changes of temperature produced 
when the first and second, first and third, and second and third bases displace one 
another, then the change of temperature arising from the first substitution should be 
equal to the difference between the changes of temperature produced by the two 
latter. A few examples will illustrate this point. 
The numbers expressing the heat developed when the nitrate of water is decom- 
posed by potash and lime are 6 0, 76 and 7°'20. The difference of these numbers is 
— 0 o, 44, indicating that a depression of temperature to that amount ought to take 
place when the former base is substituted for the latter. We have before seen that 
the result of the direct experiment is — 0 o- 37. In this and the following cases, the 
temperatures corrected only for the vessels are adopted, because I have not deter- 
mined the specific heats of the metallic solutions. The error in the comparison from 
this circumstance is wholly insignificant. 
The heat produced in two experiments in which sulphate of water was decomposed 
by potash was 7°'24 and 7°‘22. The same compound decomposed by ammonia gave 
in different trials 6 o, 40, 6 0, 53 and 6°*51. The difference of the means of these num- 
bers is +0 o, 74. The direct experiment gave in one experiment -{-0 o *75, in another 
+ 0°78. 
The corresponding number which expresses the substitution of oxide of zinc for 
water in the sulphate of water is 5°*40, and this taken from 7°'22 leaves 1 0, 82 for the 
heat due to the displacement of oxide of zinc by potash. The direct experiment gave 
]°'87. 
Two experiments were made to determine the heat arising from the substitution of 
oxide of copper for the base in the nitrate of water. In one of these experiments the 
hydrated oxide was taken ; in the other, the anhydrous oxide, obtained by precipi- 
