100 Prof. H. E. Armstrong. On Chemical Changs, [Mar. 13, 



The amount of impurity present in the gases being reduced to a 

 minimum, i.e., the gases being almost dry and almost free from 

 impurities which in admixture with water constitute a conducting system. 

 change takes place at a very slow rate when heat is applied; and 

 even when a considerable amount of water is present, the amount of 

 associated impurity is too small to raise the conductivity — the rate of 

 formation of conducting systems — to a point at which the rate of 

 change would be such as to give rise to an explosive wave. 



As defining the conditions which it has long been my opinion are 

 necessary to the occurrence of chemical change in gases and generally, 

 I may refer to words used by me in 1893.* I venture to call attention 

 to them, not because I have any particular wish to put forward a 

 claim ou my own behalf but in the hope of attracting attention to a 

 subject of surpassing importance, which both chemists and physicists 

 have hitherto most stransrelv neglected, to consider in all its bearinqs. 



* "Eight years ago, in the course of the discussion on Mr. H. B. Baker's com- 

 munication on " Combustion in Dried Gases " (these 'Proceedings,' 18S5, 40), I 

 defined chemical action as reversed electrolysis : in other words, in order that 

 chemical action may take place, it is essential that the system operated on comprise 

 an electrolyte. I then pointed out that as neither hydrogen nor oxygen was an 

 electrolyte, a mixture of only these two gases should not be explosive ; and, 

 moreover, that as water was not an electrolyte, and it was scarcely probable that 

 water and [either] oxygen or hydrogen would form an electrolyte, it was difficult 

 to understand how the presence of water pure and simple should be of influence 

 in the case of a mixture of hydrogen and oxygen. This forecast has since been 

 verified, the remarkable series of experiments carried out by Y. Meyer in conjunc- 

 tion with Erause and Askenasy having clearly demonstrated that the formation of 

 water from hydrogen and oxygen takes place at an irregular rate, and is, therefore, 

 dependent on the presence of a something other than water — I imagine an acid 

 impurity. But this is a consideration which has not yet received the proper 

 attention, and it is. therefore, desirable to emphasise its importance by reference to 

 other cases. Mr. Baker's recent preliminary note on the influence of rnoisture in 

 promoting chemical action {ante, p. 229) affords several interesting examples: — 

 Thus, he states that neither does hydrogen chloride combine with ammonia nor is 

 nitric oxide oxidised by oxygen if moisture be excluded. In the former case, 

 the addition of water should suffice to determine the combination, as water and 

 hydrogen chloride together form a ' composite electrolyte ' (cf. 1 Boy. Soc. Proc.,' 

 1S86, No. 243, p. 268) ; as neither nitric oxide nor oxygen, however, forms a 

 composite electrolyte with water, in this case water alone should not determine the 

 occurrence of change — but if by the introduction of a trace of ' impurity ' in 

 addition to water the presence of a composite electrolyte were secured (however 

 high its resistance, owing to the smallness of the amount of ' impurity '), action 

 would set in, and when once commenced would proceed at an increasing rate, as 

 nitric acid would be formed and the resistance of the electrolyte would conse- 

 quently diminish. On this account it will be a task of exceeding difficulty to 

 demonstrate experimentally that nitric oxide and oxygen are inactive in presence 

 of water alone ; but there can be no doubt that such must eventually be admitted 

 to be the case, provided always that it is permissible to extrapolate Eohlrausch's 

 observations and to conclude from them that pure water is a dielectric." — ' Chem. 

 Soc. Proc.,' 1893, 145. 



