276 Dr. van der Kolk on the Mechanical 



further combination, since, at least within certain limits, the 

 strength of chemical affinity increases with the temperature. 



In other compounds (nitric acid, for example) the components 

 possess less energy than the body formed. In such a case the 

 spark must not only cause a possible increase of affinity, but it 

 must in addition furnish the needful energy ; each spark, how- 

 ever, yields only a limited quantity of work, so that the entire 

 mass of such a mixture can never be made to combine by a 

 single spark. 



According to Favre and Silbermann*, 1 gramme of hydrogen 

 evolved 7576 units of heat by its conversion into ammonia ; 

 ammonia therefore contains less energy than its components. 

 Its behaviour with the electric spark is, however, peculiar. Ac- 

 cording to Bufff, gaseous ammonia is gradually decomposed by 

 powerful electric sparks ; but it is also stated that nitrogen and 

 hydrogen combine under the influence of the spark. But since 

 it is nevertheless impossible that the same spark should produce 

 two completely contrary effects, there must necessarily be some 

 difference between the two cases, which further researches will 

 bring to light. This calls to mind the well-known facts of the 

 decomposition of water-vapour by iron turnings, and the for- 

 mation of water when hydrogen is passed over oxide of iron J. 



2. Some .cases of so-called catalytic action receive a higher 

 explanation on this theory. Platinum causes hydrogen and 

 oxygen to combine to form water, but it does not unite nitrogen 

 and oxygen into nitrous oxide. These facts are connected with 

 the principles that have been developed above. It is evident 

 that the platinum can only modify the affinity; for inasmuch as 

 it does not itself undergo any alteration, it is impossible that it 

 should develope energy. It can only occasion the formation of 

 compound bodies in cases where the required energy is already 

 present in the constituents, and not when, as with protoxide of 

 nitrogen, the constituents contain less energy than the com- 

 pound. In like manner platinum converts ozone into oxygen, a 

 body which contains less energy, but cannot bring about the op- 

 posite change. 



It is thus explained how it happens that the electric spark 

 and platinum produce the same effect (formation of water) upon 

 explosive gas, but contrary effects upon ozone and oxygen — the 

 electric spark converting oxygen into ozone, and platinum con- 

 verting ozone into oxygen. 



It is stated that oil of turpentine, when shaken with oxygen, 



* Ann. de Chim. et de Phys. 3 ser. vol. xxxvii. 

 t Fortschritte der PhysiJc, 1860, p. 501. 



J [By an obvious oversight the original has Kupferdrahtspane (scraps of 

 copper wire) and Kupferoxyd (oxide of copper). — Transl.J 



