110 
ON BOILING WATER. 
pected ; but the above experiments seem to show that boiling is by no means necessarily 
the phenomenon that has generally been supposed, viz. a separation of cohesion in the 
molecules of a liquid from distension by heat. I believe, from the close investigation I 
made into the subject, that (except with the metals, on which there is no evidence) no 
one has seen the phenomenon of pure boiling without permanent gas being freed, and 
that what is ordinarily termed boiling arises from the extrication of a bubble of perma¬ 
nent gas either by chemical decomposition of the liquid,- or by the separation of some 
gas associated in minute quantity with the liquid, and from which human means have 
hitherto failed to purge it; this bubble once extricated, the vapour of the liquid expands 
it, or, to use the appropriate phrase of M. Donny, the liquid evaporates against the sur* 
face of the gas. 
My experiments are, in a certain sense, the complement of his. He showed that 
the temperature of the boiling point was raised in some proportion as water was de¬ 
prived of air, and that under such circumstances the boiling took place by soubresauts. 
I have, I trust, shown that when the vapour liberated by boiling is allowed to condense, 
it does not altogether collapse into a liquid, but leaves a residual bubble of permanent 
gas, and that at a certain point this evolution becomes uniform. 
Boiling, then, is not the result of merely raising a liquid to a given temperature, it 
is something much more complex. 
One might suppose that with a compound liquid the initial bubble by which evapora¬ 
tion is enabled to take place might, if all foreign gas were or could be extracted, be 
formed by decomposition of the liquid ; but this could not be the case with an elemen¬ 
tary liquid; whence the oxygen from bromine or the hydrogen from phosphorus and 
sulphur ? As with the nitrogen in water, it may be that a minute portion of oxygen, 
hydrogen, or of water, is inseparable from these substances, and that if boiled away to 
absolute dryness, a minute portion of gas would be left for each ebullition. 
With water there seems a point at which the temperature of ebullition and the quan¬ 
tity of nitrogen yielded become uniform, though the latter is excessively minute. 
The circumstances of the experiments with bromine, phosphorus, and sulphur, did not 
permit me to push the experiment so far as was done with water, but as far as it went 
the result was similar. 
When an intense heat, such as that from the electric spark or voltaic arc, is applied 
to permanent gas, there are, in the greater number of cases, signs either of chemical de¬ 
composition or of molecular change ; thus compound gases, such as hydrocarbons, am¬ 
monia, the oxides of nitrogen, and many others, are decomposed. Phosphorus in vapour 
is changed to allotropic phosphorus, oxygen to ozone, which, according to present ex¬ 
perience, may he viewed as allotropic oxygen. There may be many cases where, as with 
aqueous vapour, a small portion only is decomposed, and this may be so masked by 
the volume of undecomposed gas as to escape detection ; if, for instance, the vapour of 
water were incondensable, the fact that a portion of it is decomposed by the electric 
spark or ignited platinum would not have been observed. 
All these facts show that the effect of intense heat applied to liquids and gases is 
much less simple, and presents greater interest to the chemist, than has generally been 
supposed. In far the greater number of cases, possibly in all, it is not mere expansion 
into vapour which is produced by intense heat, but there is a chemical or molecular 
change. Had circumstances permitted I should have carried these experiments further, 
and endeavoured to find an experimentum crucis on the subject. There are difficulties 
with such substances as bromine, phosphorus, etc., arising from their action on the sub¬ 
stances used to contain and heat them, which are not easy to vanquish, and those who 
may feel inclined to repeat my experiments will find these difficulties greater than they 
appear in narration; but I do not think they are insuperable, and hope that, in the 
hands of those who are fortunate enough to have time at their disposal, they may be 
overcome. 
To completely isolate a substance from the surrounding air, and yet be able to experi¬ 
ment on it, is far more difficult than is generally supposed. The air-pump is but a rude 
mode for such experiments as are here detailed. 
Caoutchouc joints are out of the question ; even platinum wires carefully sealed into 
glass, though, as far as I have been able to observe, forming a joint which will not allow 
gas to pass, yet it is one through which liquids will effect a passage, at all events 
when the wires are repeatedly heated. 
