Affinities of the Elements of Water. 509 
than 23.2, the probable mean of the voltameter, and perhaps to be preferred to it, 
as obtained by means less complex. The mean of all, 25.1, may with tolerable 
certainty be assumed as the real value. If we might suppose this rate of decrease 
598.9 
to be uniform, /.0 would vanish at L * 100° = 2386°, about half way be- 
25. 
tween the fusing points of gold and cast iron. Judging from my recollection of 
the appearance of the platinum wire, which I saw performing the decomposition 
of water with Mr. Gassiot’s magnificent apparatus, it must actually have been 
near this ; but I hope before long to determine the precise point of the scale by 
actual measurement. 
These facts, I think, show that up to 202° the affinity of the elements of 
water is lessened by heat ; they increase the difficulty already noticed of explain- 
ing how this agent can cause the combination of those bodies; but they are, at 
least, consistent with the wide range of facts which show that the character of its 
action is essentially repulsive. They also justify a doubt whether that combina- 
tion is really produced by its direct influence, or by its evolving some other agent 
of opposite and more powerful tendency. Nor are we withoutsome obscure indi- 
cations that such actually is the case; for Sir Humphrey Davy found, in his cele- 
brated Researches on Flame, that hydrogen will not burn, nor a mixture of it 
and oxygen explode, unless acted on by a body heated so as to emit light. Ata 
temperature something lower the gases combine, without a propagation of the 
action from one part of the mixture to another, and apparently by its successive 
contact with the heated surface. In the case of chlorine, light causes the explo- 
sion at low temperatures; but the repulsive forces are feeble in this gas, as is mani- 
fest from its liquefaction at a moderate pressure. At those temperatures, too, it 
exercises a strong absorptive action on certain rays of light, which the experiments 
of Brewster prove to increase as the heat is raised. ‘The absorption of light by 
oxygen is, I believe, insensible in ordinary circumstances; but the analogies of 
chlorine and iodine authorize a belief that it may be energetic at an elevated 
temperature. Yet more, as heat, when it becomes latent, changes its relations to 
temperature for others belonging to cohesion, may not light, when it loses its 
visible character by absorption (or otherwise), acquire another connected with 
affinity ? In the slower and tranquil combustion we have evidently at work that 
capillary force of which I have already spoken, and which, in the instance of 
platina sponge, is alone sufficient to overcome the elasticity of the gases. But in 
