30 
DR. ANDREWS ON THE THERMAL CHANGES 
substitution should take place. But these conditions can rarely be fulfilled. It is 
important, however, to remark, that, with one or two exceptions, the observed devia- 
tions are in the same direction which theory would indicate. The difficulty of 
obtaining - most of the metallic salts in a perfectly neutral condition, and in a definite 
state of composition, is well known ; and in the case of deliquescent compounds, a 
separate analysis of the specimen can rarely be employed with advantag-e. The 
variable nature of the precipitate arising from imperfect substitutions is a fertile 
source of divergences in the results, and the tendency to this is further increased by 
the necessity of performing all the experiments without the application of external 
heat. The formation of a subsalt produces less heat than the precipitation of the 
hydrated oxide, for the obvious reason that in the former case an imperfect substitu- 
tion takes place. A remarkable example of this has been already cited in the action 
of potash on the acetate of zinc, where a great deviation from the usual development 
of heat is distinctly shown to depend upon the precipitate being a subsalt. The same 
cause, no doubt, frequently interferes with the accuracy of the result in other cases, 
where only a small portion of subsalt is formed. In other examples, part of the 
original base still remains in solution ; and in others again, portions of the substitu- 
ting base are carried down along with the precipitate. When we take into conside- 
ration all these sources of error, the numerous instances of perfect agreement with, 
as contrasted with the few examples of slight deviation from, the general law of the 
heat of substitution being equal for the same bases, appear to be sufficient fully to 
establish its accuracy. 
It may be here observed that the accuracy of this principle will not be in any way 
affected whatever views may be adopted, as to the exact changes which occur when 
one base displaces another. Whether we consider the final result to arise from the 
simple substitution of one base into the position occupied by another, or from a series 
of distinct chemical changes, each producing a certain thermal effect, the general 
facts now established will not be the less rigorously true. 
The separation of most of the bases in a solid form, will of course tend to produce 
heat, and as this will vary with different precipitates, the numbers for the insoluble 
bases cannot be exactly compared with one another. The amount of the latent heat 
due to precipitation is unknown, but it must be the same whenever the same preci- 
pitate is formed. The correction for this cause will, therefore, be a constant quantity 
for the salts of the same base, and if applied, could not affect the equality of the 
foregoing groups of numbers. It is important to observe, that, notwithstanding the 
heat due to the formation of the precipitates, a diminution of temperature occurs 
when potash is substituted for lime or magnesia. 
On a first view this last fact appears to prove that potash is a less powerful thermal 
base than lime or magnesia, but a closer examination will show that it is at least 
premature to draw such a conclusion. It must be remembered that we are imper- 
fectly acquainted with all the chemical changes which accompany the substitutions 
