272 Dr. van der Kolk on the Mechanical 



heat when decomposed ; they break up when heated, and are not 

 reproduced on cooling. 



The following additional examples may be quoted from 

 Deville's paper referred to above — namely, chloride, iodide, and 

 sulphide of nitrogen, which are suddenly decomposed with evo- 

 lution of heat by elevation of temperature, but are not reproduced 

 by subsequent cooling. The law above stated holds good also 

 for the transformations' of dimorphous or polymorphous sub- 

 stances from one condition to another. 



1. When Arragonite is heated, it changes into calc-spar and 

 evolves heat; Arragonite therefore contains more energy than 

 calc-spar, and hence the opposite change does not take place on 

 cooling. 



2. Crystals of sulphur obtained by fusion (and belonging to 

 the Fifth System) change at a low temperature, with evolution 

 of heat, into crystals of the same form as those of native sulphur 

 (which belong to the Fourth System). Again, therefore, more 

 energy is contained in the substance in its first condition than 

 in the second, and accordingly the inverse process does not 

 occur at low temperatures. 



3. Plastic sulphur (soufre mou) likewise evolves heat while 

 passing into ordinary sulphur, and hence contains more energy 

 than this. Ordinary sulphur therefore cannot of- itself change 

 into the plastic modification, but the opposite change can occur. 



4. Deville* speaks of a third modification of sulphur, which 

 is insoluble in sulphide of carbon. By warming, this variety is 

 converted into common sulphur ; and it follows from the expe- 

 riments of Fordos and Gelisf, that it takes up heat during the 

 change, thus exhibiting the opposite behaviour to that of plastic 

 sulphur. This insoluble modification accordingly contains less 

 energy than ordinary sulphur, and apparently it does not change 

 spontaneously into the latter. DevilleJ kept some fragments at 

 the ordinary temperature, which were unchanged after six years. 



Herein lies the explanation of the following observations by 

 Favre§. He finds that in the formation of sulphurous acid from 

 ordinary sulphur, and its further oxidation to sulphuric acid by 

 means of chlorine, each equivalent evolves 67,212 thermal units 

 (the equivalent of hydrogen being taken as 1 gramme), whereas 

 by direct oxidation by means of hypochlorous acid only 64,110 

 thermal units are evolved. In the last case, however, the less 



the above-named compounds do not form of themselves after decomposi- 

 tion ; but he merely deduces this from the observed phenomena, and does 

 not give it as in any way a consequence of a general theory. 



* Ann. de Ckim. et de Phys. 3 ser. vol. xlvii. p. 94. 



t Ibid. p. 108. t Ibid. p. 100. 



§ Theses, &c, p. 43. 



