288 



Prof. H. E. Armstrong. Electrolytic [Mar. 25, 



in constitution between water and the bodies fonnd incapable of 

 determining explosion : the former being a compound of a positive 

 with a negative element, the latter being all compounds of two nega- 

 tive elements ; and if it be permissible to generalise from this single 

 instance, it may hence be stated, that in order that interaction shall 

 take place in cases such as that under consideration, it is not only 

 necessary that the elements of the " catalyst " shall be divisible*' 

 between the interacting substances — the v elements of C0 2 are obviously 

 as divisible between CO and 2 as are those of H 2 — but that the 

 catalyst shall consist of a positive and a negative and not of two 

 negative radicles. On this view, it is possible to understand that 

 water itself may act as the catalyst in determining the formation of 

 water at high temperatures from hydrogen and oxygen. j 



In the case discussed (the oxidation of carbon monoxide), inter- 

 action takes place at a very high temperature, and therefore — since 

 high temperature may be regarded as the equivalent of high electro- 

 motive force — under conditions under which the catalyst water is 

 probably a simple electrolyte. The behaviour of sulphur dioxide 

 in presence of oxygen and water is instructive as being a case of an 

 analogous interaction occurring at a low temperature. From a most 

 carefully conducted series of experiments by Mr. Dixon (" Journ. of 

 Gras Lighting," 1881, 37, p. 704), it appears that not only does sulphur 

 dioxide not undergo change in contact with dry oxygen, but that it 

 even resists oxidation if water vapour be present and at a temperature 

 of 100° ; as is well known, however, oxidation takes place — but only 

 slowly — when an aqueous solution of sulphur dioxide is in contact 

 with oxygen. In this case, in the gaseous mixture the water appa- 

 rently is not under such conditions that it can act as a simple electro- 

 lyte, or even form a composite electrolyte, and action only takes place 

 when the conditions become such that a composite electrolyte can be 

 formed ; Ostwald's observations may be held to prove, I imagine, that 

 a very imperfect composite electrolyte results on dissolving sulphur 

 dioxide in water, and in accordance with this is the fact that the 

 aqueous solution is but slowly oxidised.^ 



* Mr. Dixon's experiments appear to prove that during the interaction of carbonic 

 oxide and oxygen in presence of water an actual division of the elements of the water 

 molecules takes place between the carbonic oxide and oxygen molecules, and hence 

 that the water does not exercise a mere contact action. 



f When this question first came under discussion at the Chemical Society, 

 I said that I looked forward to the time when probably it would be found that a 

 mixture of pure hydrogen and oxygen was inexplosive, like one of pure carbonic 

 oxide and oxygen. I was then still under the influence of current opinion and 

 regarded water as a saturated compound, and had not yet realised the important 

 function of " residual affinity " in such changes. 



X The behaviour here described of sulphur dioxide appears to me to furnish 

 another argument adverse to the dissociation hypothesis, as oxidation takes place 

 under the conditions least favourable to the occurrence of dissociation. 



