64 REPORT—1849, 
As the subject is of great extent and the inquiry has hitherto embraced 
only a small number of cases of solution, it is not unlikely that some of these 
conclusions will require hereafter to be modified. From some experiments 
by Graham on the solution of salts belonging to certain isomorphous groups, 
there is reason to suspect the existence of a connexion between isomorphism 
and the absorption of heat in solution. 
The foregoing remarks apply only to the solution of crystallized salts. If, 
however, we take a salt which crystallizes with water and make it anhydrous 
before solution, the thermal results will be altogether different. The anhy- 
drous salt, when added to an excess of. water, will first combine with its ordi- 
nary equivalent of water of crystallization, and the new compound will then 
dissolve. ‘The change of temperature observed is therefore a complex quan- 
tity arising from the heat of combination due to the union of the anhydrous 
salt with water, and the heat absorbed by the solution of the hydrous salt. 
From a comparison of the results obtained on dissolving the same salt in the 
anhydrous and hydrous states, Graham has endeavoured to deduce the amount 
of heat due to the combination of the dry salt with its water of crystallization. 
According to his experiments, the sulphates of water, copper and manganese, 
disengage the same quantity of heat in combining with the first atom of © 
water. The sulphates of magnesia and zinc also disengage equal quantities 
of heat in their complete hydration. The same simple relation is not however 
observed to hold between the quantities of heat evolved in the complete hy- 
dration of the first set of salts, or in the combination of the second set with 
the first atom of water. Neither does it apply to the other sulphates of the 
magnesian series, 
~ None of the experiments hitherto published furnish all the requisite data 
for calculating with precision the absolute quantities of heat set free or 
absorbed in these cases of chemical action. The weights of the water and of 
the salt are given, and sometimes the weight and form of the vessel, and the 
material of which it is composed; but these data are not sufficient to enable 
us to deduce the true numbers from the observed increments or decrements 
of temperature. Knowing the weight and composition of the containing 
vessel, we may, it is true, calculate its thermal value in water. But other 
corrections, such as those for the heating and cooling influence of the sur- 
rounding air, can only be ascertained by special experiments performed 
under similar conditions to the original observations. Neither have any ex- 
periments of sufficient accuracy been made to determine the specific heats of 
the solutions formed. 
To complete an investigation which would furnish all these elements, would 
be a work of very great labour, and will probably scarcely be undertaken till 
our instruments and means of observation are greatly improved. As a first 
step to such an inquiry, I may here describe a few preliminary experiments 
on the specific heats of some saline solutions, and on the quantities of heat 
absorbed in the solution of successive portions of the same salt, 
To obtain results approaching to accuracy in experiments on the specific 
heats of saline solutions is extremely difficult, as the errors of experiment are 
often of nearly the same order of magnitude as the whole differences to be 
observed. The corrections for the cooling and heating action of the air and ~ 
for the effects of radiation, cannot be estimated with any certainty by the ap- 
plication of general formulas founded on experiments made at a different 
time* ; and the most careful examination of the calibre of the thermometer 
lr i i a me 
a 



7 
i 
i 
‘ 
* If the vessel be uncovered, changes in the hygrometric state of the atmosphere produce 
a very marked influence on the rate of cooling, when the excess of temperature above the air — 
