Dr. Andrews's lleport on the Heat of Combination, 521 



The same is also well known to be true of the hydrocj'anic and car- 

 bonic acids. In the case of bases, such as the oxide of copper, whose 

 salts have all an acid reaction, this criterion will not apply ; but 

 the exceptional acids are so iew, and their peculiarities so well- 

 marked, that they give rise to little difficulty in the experimental in- 

 vestigation. 



.**'The quantities of heat developed by different bases in combining 

 "With the same acid are so different, that it is unnecessary to refer 

 particularly to the proofs of the second law. In this case, neutrali- 

 zing power has no apparent influence on the results, as oxide of silver, 

 which forms salts neutral to test paper with the strongest acids, is 

 one of the feeblest bases if measured by its thermal power. It de- 

 velopes, in fact, little more than one-third of the heat which potash 

 does in combining with the acids. 



The more recent experiments of Graham and of Fabre and Sil- 

 bermann, confirm the accuracy of the facts from which the second 

 and third laws were deduced, that no heat is developed on mixing 

 solutions of neutral salts or of a neutral salt and acid*. It is diffi- 

 cult however to obtain, as Graham has remarked, positive proof of 

 the occurrence of combination, when sucli solutions are brought into 

 contact. Fabre and Silberniann indeed are of opinion that acid salts 

 cannot exist in the state of solution. 



Double Decompositions. — When solutions of two neutral salts are 

 mixed and a precipitate formed from their mutual decomposition, 

 there is always a disengagement of heat, which, though not consi- 

 derable, is perfectly definite in amount. It does not altogether arise 

 from the components of the precipitate having changed from the 

 fluid to the solid state — as it is not always the same for the same 

 precipitate — but it is chiefly connected with the latent heat of the 

 precipitate. If the latter contains water of crystallization, the heat 

 given out is much greater than when an anhydrous precipitate is 

 formed. Experiments of this kind appear at first view to be ex- 

 tremely simple, but it is often difficult to obtain exact results, from 

 the length of time during which the heat continues to be disengaged, 

 even when the combination is aided by brisk agitation. .' 



The precipitation of the salts of barytes and lead by a Soluble 

 sulphate appeared to present favourable conditions for investigation, 

 and accordingly I made an extensive set of experiments with these 

 classes of salts. This is indeed the only part of the inquiry which I 

 have been able to complete. A few other examples of double de- 

 composition will however be noticed. '[' 



Chloride of Barium and Sulphate of Magnesia. — Of chloride &f 

 barium carefully purified and dried immediately before the experi- 

 ment at a low red heat, 16'94< grms. were taken in each experiment, 

 equivalent to 19'00 grms. sulphate of barytes. The weight of sul- 

 phate of magnesia (dry) was 10*3 grms., which is a little more than 



* Slight changes of temperature may however occasionally be detected ; but in 

 some cases a development, in others, an absorption of heat occurs. These thermal 

 effects evidently arise from causes altogether distinct from those which produce the 

 combination of acids and bases. 



Phil, Mag, S. 3. No. 246. Suppl. Vol. 36. 2 M 



