THEORY OF THE COMBINATION OF GASES BY PLATINA. 69 



any difficulty which might arise on that ground. Sir James Hall found carbonic 

 acid and lime to remain combined under pressure at temperatures at which they 

 would not have remained combined if the pressure had been removed ; and I have 

 had occasion to observe a case of direct combination in chlorine*, which being 

 compressed at common temperatures will combine with water, and form a definite 

 crystalline hydrate, incapable either of being formed or of existing if that pressure 

 be removed. 



630. The course of events when platina acts upon, and combines oxygen and hy- 

 drogen, may be stated, according to these principles, as follows. From the influence of 

 the circumstances mentioned (619. &c.), i. e. the deficiency of elastic power and the 

 attraction of the metal for the gases, the latter, when they are in association with the 

 former, are so far condensed as to be brought within the action of their mutual affini- 

 ties at the existing temperature ; the deficiency of elastic power, not merely subject- 

 ing them more closely to the attractive influence of the metal, but also bringing them 

 into a more favourable state for union, by abstracting a part of that power (upon 

 which depends their elasticity,) which elsewhere in the mass of gases is opposing their 

 combination. The consequence of their combination is the production of the vapour 

 of water and an elevation of temperature. But as the attraction of the platina for 

 the water formed is not greater than for the gases, if so great, (for the metal is scarcely 

 hygrometric,) the vapour is quickly diff'used through the remaining gases ; fresh por- 

 tions of the latter, therefore, come into juxtaposition with the metal, combine, and 

 the vapour formed is also diffused, allowing new portions of gas to be acted upon. In 

 this way the process advances, but is accelerated by the evolution of heat, which is 

 known by experiment to facilitate the combination in proportion to its intensity, and 

 the temperature is thus gradually exalted until ignition results. 



631. The dissipation of the vapour produced at the surface of the platina, and the 

 contact of fresh oxygen and hydrogen with the metal, form no difficulty in this expli- 

 cation. The platina is not considered as causing the combination of any particles 

 with itself, but only associating them closely around it ; and the compressed particles 

 are as free to move from the platina, being replaced by other particles, as a portion 

 of dense air upon the surface of the globe, or at the bottom of a deep mine, is free to 

 move by the slightest impulse into the upper and rarer parts of the atmosphere. 



632. It can hardly be necessary to give any reasons why platina does not show this 

 effect under ordinary circumstances. It is then not sufficiently clean (617.)> ^^^ 

 the gases are prevented from touching it, and suffering that degree of effect which is 

 needful to commence their combination at common temperatures, and which they can 

 only experience at its surface. In fact, the very power which causes the combination 

 of oxygen and hydrogen is competent, under the usual casual exposure of platina, to 

 condense extraneous matters upon its surface, which soiling it, take away for the time 

 its power of combining oxygen and hydrogen by preventing their contact with it. 



* PhUosophical Transactions, 1823, p. 161. 



