22 
DR. ANDREWS ON THE THERMAL CHANGES 
mixed together, is a question of very great difficulty, requiring the previous determi- 
nation of many data, all of which are in few instances capable of being discovered by 
experiment. The liquids before mixture are, in fact, solutions of the acid and alkali 
in the state of hydrates, and as large quantities of heat were evolved during the 
formation of these hydrates, and generally also during their subsequent solution, an 
equal absorption of heat will take place when these combinations are destroyed. A 
further allowance is also required for the effect of the solution of the resulting com- 
pound. After making all these corrections it is doubtful whether the result finally 
obtained would not be a theoretical or imaginary number. 
If we adopt the view, now generally admitted by chemists, that the hydrated acids 
are in reality compounds of the pure acids with water performing the functions of a 
base, the heat produced when a dilute acid is neutralized by a base will arise from 
the latter displacing an equivalent' of basic water*, and the general result, before re- 
ferred to, may be thus expressed. “When the same base displaces water from any 
of its acid combinations, the heat disengaged is nearly the same.” If for basic water 
we substitute any base, the law will receive a very general form as follows : — 
When one base displaces another from any of its neutral combinations , the heat evolved 
or abstracted is always the same, whatever the acid element may be, provided the bases 
are the same. 
The following experiments were undertaken for the purpose of determining the 
accuracy of this principle. The base selected to displace others was the hydrate of 
potash, and it was always employed in a state of dilute solution. The strength of the 
solution was ascertained by neutralizing a determinate quantity with sulphuric acid 
of known strength. The required quantity was weighed in a thin brass vessel of a 
long cylindrical form, coated externally with copal varnish. The latter precaution 
effectually protected it from the action of all metallic solutions. The equivalent 
solution of the salt to be decomposed was contained in a thin glass jar, supported 
within a larger one, by means of a projecting rim. The whole was so adjusted that 
when the brass vessel with its contents was cautiously placed in the saline liquid it 
was sufficiently buoyant to float, and, at the same time, it extended through nearly 
the entire depth of the liquid. The weight of the two liquids taken together was 
1000 grains, of which the saline solution formed about 700 grains. To bring the 
two liquids to the same temperature, a rapid rotatory motion was given to the inner 
vessel, by moving a light glass stirrer round in it. In the outer vessel a very delicate 
thermometer with a long cylindrical bulb was suspended. As soon as a perfect equi- 
librium of temperature was established between the two fluids, the position of the 
thermometer was carefully observed. The edge of the brass vessel was then grasped 
with pincers, and its contents quickly added to the saline solution. The mixture was 
now rapidly stirred and the new position of the thermometer noted. The brass vessel 
was not again introduced into the liquid after the mixture had taken place. 
* Transactions of the Royal Irish Academy, v. xix. 247. 
