Energy of Chemical Actions. 273 



energetic insoluble modification was employed ; and since in this 

 experiment the components contained less energy than in the 

 former one, but the resulting sulphuric acid the same quantity 

 in both cases, the difference between the energy of the reagents 

 and that of the products, that is, the heat evolved, was less in the 

 second case. 



5. It is pointed out in the same memoir (Favre, Theses, fyc, 

 p. 25) that 1331 units of heat are absorbed in the conversion 

 of opake arsenious acid into the vitreous modification. Hence 

 more energy exists in the second modification than in the first ; 

 and in reality vitreous arsenious acid changes spontaneously 

 into the opake variety, but the opposite change takes place only 

 on heating. 



6. Red phosphorus possesses more energy than common yel- 

 low phosphorus* ; nevertheless it does not of itself change into 

 the latter. This, however, does not at all contradict the theorem ; 

 for this does not say that the transformation will occur in every 

 case, but only that, if transformation does take place, the body 

 passes into a state in which it contains less energy. 



7. Ozone also may be quoted as an example. It contains 

 more energy than ordinary oxygen, into which it is changed by 

 heating, no doubt evolving heat at the same time. Subsequent 

 cooling does not reproduce the ozone. 



In accordance with the foregoing facts, it may be established 

 as a general theorem, that — 



When a body on heating passes from one condition to another 

 with evolution of heat, it does not return to its first condition 

 upon subsequent cooling. 



The examples already given may suffice to illustrate this 

 theorem. It is possible that exceptions will come to light; but 

 if so, they must be such as can be referred to the two cases stated 

 above. It is of course understood that the chemical affinity 

 between molecules must also be taken into account, and we can 

 imagine this sufficiently powerful to draw from surrounding 

 objects the energy required for the combination. Something of 



* [There appears to be a mistake here : according to Favre's experiments, 

 as reported in Liebig and Kopp's Jahresbericht for 1853, p. 24, the trans- 

 formation of 31 grammes of red phosphorus (1 gramme-atom) into com- 

 mon phosphorus is attended with the absorption of 28,246 thermal units. 

 This result, taken in connexion with the comparative ease with which yel- 

 low phosphorus changes into the red modification, harmonizes better with 

 the principles which the author is seeking to establish, than his own state- 

 ment contained in the text. In Miller's ' Elements of Chemistry ' (3rd ed. 

 vol. ii. p. 220), however, it is stated that when red phosphorus is heated to 

 the temperature at which it is converted into yellow phosphorus, " the 

 whole mass suddenly passes back into the ordinary form, with a copious 

 evolution of heat." — Transl.] 



