70 REPORT—1849. 
unavoidable exception to this statement, and in the experiments with them, 
the results would require to be corrected for the heat due to the change of 
the base from the solid to the fluid state As this correction, however, al- 
though unknown, must be a constant quantity for the same base, it would 
not, if applied, interfere with the direct proof of the first law. 
In an inquiry of this kind, it is important, while endeavouring to generalize 
the results of experiment, to point out at the same time the differences which 
occur in particular cases between those results and the numbers deduced from 
the theory. In the whole range of the science of heat, scarcely a single . 
general principle has yet been discovered which is strictly in accordance 
with all the results of experiment ; and from the application of improved me- 
thods of experimenting, discrepancies of this kind have of late years been 
found to exist where they had not before been suspected. 
In the original experiments from which the first of the foregoing laws was 
deduced, the mean heatdeveloped by the nitric, phosphoric, arsenic, hydrochlo- 
ric, hydriodic, boracic, chromic and oxalic acids being 6°61, the greatest de- 
viation from the mean on either side amounted only to 0%15; and a similar 
remark may be made with respect to the combinations of soda, barytes and 
ammonia. On the other hand, sulphuric acid disengaged about 0*7 more,than 
the mean quantity, and the citric, tartaric and succinic acids about 0%5 less. 
To ascertain whether these discrepancies depended on the state of dilution of 
the solutions, I repeated these experiments lately with solutions of only half the 
strength, but although only half the heat was obtained, similar differences 
were still found to exist. If, instead of taking just the quantity of sulphuric 
acid required to neutralize the base, we employ a large excess, the heat given 
out during combination will be nearly 0°-2 less, which reduces the anomaly 
presented by this acid to about 0°5. The sulphurous acid not having been 
formerly examined, I have lately made some experiments on its thermal re- — 
lations to the bases, the results of which are very interesting. Although one 
of the feeblest acids, it agrees almost exactly with sulphuric acid in the heat 
developed by its combination with potash. In several carefully conducted 
experiments the increments of temperature did not differ more than 0°05. 
Combining this with the fact that acids differing so much in composition and 
properties as the nitric, boracic and oxalic, also disengage almost exactly the 
same amount of heat in the act of combination, there will, [ conceive, be 
little hesitation in attributing the deviations already mentioned to the in- 
fluence of extraneous causes, and in acknowledging the truth of the principle, 
that the heat of combination depends upon the neutralization or combination 
of the base, and not upon the nature of the acid by which the base is neutral- 
ized. That other causes of change of temperature, of feeble power, do ac- 
tually exist, may be proved by the following fact. If we add an excess of 
sulphuric acid to the neutral solution after combination has taken place, a 
slight fall of temperature, amounting to about 0%1, will occur; if we make 
the same experiment with sulphurous acid, an increase of temperature of about 
equal amount will be observed, while with oxalic acid there will be no 
thermal change of any kind. Now it is very probable that the same causes 
which produce these slight thermal effects are in operation during the original 
combination of the acid and base, and if so, they would introduce anomalies 
into the quantities of heat then developed. 
There is one important condition, which, as far as my investigations ex- 
tend, requires to be fulfilled in order that the first law may hold good; viz. 
the acid must have the power of neutralizing the alkaline reaction of the 
bases. It is for this reason that the hydrocyanic, carbonic and arsenious 
acids do not develope the same quantity of heat in combining with potash as 
ee a a 

