So CORRELATION OF PHYSICAL FORCES. 



netic effects, and also of forming and decomposing chemical 

 compounds, and in some proportion to the progression of heat : 

 this has not, indeed, as yet been proved to bear a measurable 

 quantitative relation to the other forces thus produced by it, 

 because so little of the heat is utilised or converted into electri- 

 city, much being dissipated, without change in the form of heat. 



Heat, however, directly affects and modifies both the mag- 

 net and chemical compounds ; the union of certain chemical 

 substances is induced by heat, as, for instance, the formation 

 of water by the union of oxygen and hydrogen gases : in other 

 cases this union is facilitated by heat, and in many instances, 

 as in ammonia and its salts, it is weakened or antagonised. 

 In many of these cases, however, the force of heat seems more 

 a determining than a producing influence ; yet to be this, it 

 must have an immediate relation with the force whose reaction 

 it determines : thus, although gunpowder, touched with an 

 ignited wire, subsequently carries on its own combustion or 

 chemical combination, independently of the original source of 

 heat, yet the chemical affinities of the first portion touched 

 must be exalted by, and at the cost of, the heat of the wire ; 

 for to disturb even an unstable equilibrium requires a force in 

 direct relation with those which maintain the equilibrium. 



Shortly after the first edition of this essay was published, I 

 communicated to the Royal Society some experiments by 

 which an important exception to the general effect of heat on 

 chemical affinity is removed, and the results of which induce 

 a hope that a generalised relation will ultimately be established 

 between heat, chemical affinity, and physical attraction. I 

 find that if a substance capable of supporting intense heat 

 and incapable of being acted upon by water or either of its 

 elements such a substance, for instance, as platinum, or iri- 

 dium be raised to a high point of ignition and then im- 

 mersed in water, bubbles of permanent gas ascend from it, 

 which on examination are found to consist of mixed oxygen 

 and hydrogen in the proportions in which they form water. 

 The temperature at which this is effected is, according to Dr. 

 Robinson, who has since written a valuable paper on the 

 subject, = 2 3 86. Now, when mixed oxygen and hydrogen 

 are exposed to a temperature of about 800, they combine 



