520 Dr. Andrews's Report on the Heat of Combination. 



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 wliether these discrepancies depended on the state of 

 dilution of the solutions, I repeated these experiments lately with solu- 

 tions 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 combina- 

 tion 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 ther- 

 mal relations to the bases, the results of which are very interesting. 

 Although one of the feeblest acids, it agrees almost exactly with sul- 

 phuric acid in the heat developed by its combination with potash. 

 In several carefully conducted ex'periments the increments of tem- 

 perature 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, I conceive, 

 be little hesitation in attributing the deviations already mentioned 

 to the influence of extraneous causes, and in acknowledging the truth 

 of the principle, that the heat of combination depends upon the neu- 

 tralization or combination of the base, and not upon the nature of 

 the acid by which the base is neutralized. That other causes of 

 change of temperature, of feeble power, do actually 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°'l, 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 investiga- 

 tions extend, 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 hy- 

 drocyanic, carbonic and arsenious acids do not develope the same 

 quantity of heat in combining with potash as the other acids. The 

 sparing solubility of the arsenious acid in water prevents an accu- 

 rate examination of its thermal reactions ; but on repeated trials 

 I obtained 0°*25 F., on combining with it the same quantity of pot- 

 ash which under similar conditions gave 0°'34' with nitric acid. 

 Although a considerable excess of arsenious acid was taken, as 

 proved by the fact that further additions produced no new deve- 

 lopment of heat, the solution still exhibited an alkaline reaction. 



