236 Johnston and Adams — High Pressures on the 



the quantities of hydrogen and oxygen are absolutely infinitesi- 

 mal, and become appreciable only at high temperatures. On the 

 other hand hydrogen and oxygen may be left in contact for a 

 practically unlimited time, and show absolutely no signs of 

 union; the reaction begins only at fairly high temperatures, 

 when its velocity becomes appreciable. 



This distinction between the theoretical state of equilibrium 

 on the one hand, and the state actually attained under a given 

 set of conditions and its dependence upon the reaction velocity 

 under those conditions, on the other hand, is often lost sight of. 

 Indeed, in a very large number of cases, we may say that the 

 reaction velocity is the decisive factor in determining the final 

 state, though of course the direction in which the reaction 

 proceeds — if it proceeds at all — is determined by the energy 

 relations of the various substances involved under the specific 

 conditions of the experiment. 



In some instances, so-called catalytic agents effect a great 

 increase in the reaction velocity ;* and some catalyst might be 

 present in any system which was being compressed, and so 

 bring about the reaction. But it appears unlikely that 

 pressure itself will in general act as a catalytic agent. Whether 

 pressure affects the reaction velocity in any direct way is a 

 question upon which there is, so far as the writers are aware, 

 practically no experimental evidence ; but it is probable that 

 its influence is generally negligible as compared with the 

 enormous acceleration produced by change of temperature. 

 It appears also that it may be in either direction, although 

 only in a characteristic direction for any one reaction. In one 

 case, the inversion of cane sugar by hydrochloric acid, 

 Rontgen's experiments! indicate that a pressure of 500 atm. 

 causes some diminution of velocity of this reaction. 



The general conclusion stated above is in harmony with 

 geological evidence. For instance, in nature glasses occur 

 which are certainly less stable than the corresponding crystal- 

 line form ; and we occasionally find intergrowths of two 

 modifications of the same substance which cannot in general 

 possess equal stability. Yet such cases occur in spite of the 

 fact that the systems have been subject to compression — and 

 even to some degree of temperature, in addition, — for geolog- 

 ical periods % 



As further direct evidence it may be mentioned that no 



* Thus palladium black brings about the union of oxygen and hydrogen 

 around 100°. 



fAnn. Phys.,xlv, 100, 1892. 



% The fact may be recognized, however, that the older volcanic glasses 

 have crystallized more or less completely ; a fact which indicates that even 

 under nearly ordinary conditions the process of devitrification can take place 

 with extreme slowness. 



