
ON THE HEAT OF COMBINATION. 63 
rous epoch, or even somewhat subsequently to that period, militating against 
_ the probability, that a larger amount of carbonic acid may have been present 
in the atmosphere, and diffused through the waters of the sea and rivers, 
than is found, either in the one or in the other, at the present time; nor is 
there anything to prevent us from imagining, that the absorption of carbon by 
vegetables, and the consequent rapidity of their growth, may, at least within 
certain limits, have borne some proportion to the greater amount of carbonic 
acid assumed to have been present at earlier periods in the history of our 
globe, although whether this be actually the case, is a point which I hope to 
be able hereafter to settle more to my satisfaction, as well as to report the 
results arrived at on some future occasion. 
Report on the Heat of Combination. 
- By Tuomas Anprews, M.D., F.R.S., M.R.LA. 
THERE are few molecular changes in the condition of matter which are 
not accompanied by the evolution or absorption of heat. The quantity of 
heat which is thus set free or absorbed, bears always a definite relation to 
the amount of the mechanical or chemical action, and its determination in 
each particular case is a problem of considerable interest as affording a 
measure of the forces in action. If we consider the great number of pheeno- 
mena, mechanical, electrical and chemical, among which the production of 
heat forms the only bond of connexion which has hitherto been clearly 
ascertained, although there may be strong grounds for suspecting them to be 
only modified forms of the action of the same force, the importance of inves- 
tigations of this kind to the future progress of physical science will become 
at once apparent. 
The object of the present Report is to give a general view of the actual 
state of knowledge on the subject of thermo-chemistry, under which we may 
conveniently include a description of the thermal effects that occur in che- 
mical actions of every kind. A few new experiments will be described in 
their proper places. These will be given in some detail, but when referring 
to experiments already published, all numerical quantities will, as far as pos- 
sible, be avoided. 
Before entering upon the consideration of chemical combinations and de- 
compositions properly so called, it may be useful briefly to refer to the ther- 
mal changes which accompany solution. The earlier experiments on this 
‘subject having been made solely with the object of discovering frigorific 
mixtures, do not furnish quantitative measures of any scientific value. But 
of late years the inquiry has been pursued in a more useful way by Gay- 
Lussac, Thomson, Karsten, Chodnew and Graham. The salts examined have 
been chiefly the soluble sulphates, nitrates and chlorides, and the solvents 
pure water and saline and acid solutions. The principal results of these in- 
vestigations I have endeavoured to express in the following propositions :— 
1. The solution of a crystallized salt in water is always accompanied by 
an absorption of heat. 
_ 2. If equal weights of the same salt be dissolved in succession in the same 
_ liquid, the heat absorbed will be less on each new addition of salt. 
' $. The heat absorbed by the solution of a salt in water holding other salts 
dissolved, is generally less than that absorbed by its solution in pure water. 
. 4. The heat absorbed by the solution of a salt in the dilute mineral acids, 
is generally greater than that absorbed by its solution in water. 
