280 Dr. van der Kolk on the Mechanical 



pletely than oxide of copper, and can even displace the latter 

 oxide from its combinations, actions which are dependent on 

 affinity; its heat of combination is, nevertheless, smaller than 

 that of oxide of copper*. It follows from this, that it is not 

 possible to deduce one from another the force of affinity and 

 the heat of combination ; we have in fact here to deal with two 

 magnitudes of totally distinct kinds — energy and affinity ; and 

 to take one as a measure of the other, seems to me no more 

 allowable than to take the electromotive force of the closed gal- 

 vanic circuit as equal to the tension of the electrodes when the 

 circuit is open. The heat of combination, denoting the differ- 

 ence between the quantities of energy contained in the reagents 

 and products, is a measure of the stability. Waterf, with the 

 heat of combination 29,413, is more stable than hydrochloric 

 acid, whose heat of combination is 23,783. Decomposition can 

 take place only if a quantity of energy equivalent to the heat of 

 combination is restored to the compound; and accordingly 

 decomposition may sometimes be possible in the case of hydro- 

 chloric acid, under circumstances in which it is not possible for 

 water. But it does not follow from this that decomposition will 

 actually take place every time this condition is fulfilled; che- 

 mical affinity comes also into account, as is shown by the exam- 

 ple of explosive gas, where the requisite energy is present in full 

 measure, but no combination occurs at the ordinary temperature. 

 If the heat of combination must thus be regarded as only a 

 very imperfect measure of the force of affinity, it is impossible 



* [None of these examples can be taken as conclusive that the heat of 

 chemical action ought not to be regarded as the measure of the chemical 

 affinity concerned. This will be evident if we examine each in succession. 



1 . According to Favre and Silbermann, one equivalent of potash evolves 

 16,083 units of heat by combination with 1 equiv. of sulphuric acid, and 

 17,766 units of heat by combination with 1 equiv. of phosphoric acid (to 

 form the dipotassic phosphate ?) ; according to Andrews, however, the 

 quantities of heat evolved are respectively 15,900 and 14,200. But, ad- 

 mitting Favre and Silbermann's results to be probably the most accurate, 

 it would be difficult to prove that sulphuric acid does completely displace 

 phosphoric acid from phosphate of potash. 



2. Why is potash called a stronger base than lime ? If in some cases it 

 displaces lime from its combinations, it is in other cases itself displaced by 

 lime ; moreover the remarks made upon the next example apply here also. 



3. The fact that oxide of silver can sometimes displace oxide of copper 

 from its salts is not more surprising than that it can displace oxide of hy- 

 drogen. The change is in both cases inadequately stated as the mere 

 replacement of one oxide by another; it ought rather to be regarded as 

 a double decomposition in which oxide of silver is decomposed and oxide of 

 copper is formed, — a change which corresponds to the evolution of 15,772;f 

 units of heat, and may therefore well compensate a possible absorption of 

 heat in other parts of the process. — Tbansl.] 



f Debray, Lecons de Chim. et de Phys. 1861, p. 63. 



