GASEOUS AND LIQUID STATES OE MATTER. 
589 
changes of volume with flickering movements when the temperature or pressure was 
changed in the neighbourhood of those points. The critical points of some of these 
bodies were above 100°; and in order to make the observations, it was necessary to bend 
the capillary tube before the commencement of the experiment, and to heat it in a bath 
of paraffin or oil of vitriol. 
The distinction between a gas and vapour has hitherto been founded on principles 
which are altogether arbitrary. Ether in the state of gas is called a vapour, while sul- 
phurous acid in the same state is called a gas ; yet they are both vapours, the one 
derived from a liquid boiling at 35°, the other from a liquid boiling at —10°. The dis- 
tinction is thus determined by the trivial condition of the boiling-point of the liquid, 
under the ordinary pressure of the atmosphere, being higher or lower than the ordinary 
temperature of the atmosphere. Such a distinction may have some advantages for 
practical reference, but it has no scientific value. The critical point of temperature 
affords a criterion for distinguishing a vapour from a gas, if it be considered important 
to maintain the distinction at all. Many of the properties of vapours depend on the 
gas and liquid being present in contact with one another ; and this, we have seen, can 
only occur at temperatures below the critical point. We may accordingly define a 
vapour to be a gas at any temperature under its critical point. According to this defi- 
nition, a vapour may, by pressure alone, be changed into a liquid, and may therefore 
exist in presence of its own liquid ; while a gas cannot be liquefied by pressure — that is, 
so changed by pressure as to become a visible liquid distinguished by a surface of demar- 
cation from the gas. If this definition be accepted, carbonic acid will be a vapour below 
31°, a gas above that temperature; ether a vapour below 200°, a gas above that tempe- 
rature. 
We have seen that the gaseous and liquid states are only distant stages of the same 
condition of matter, and are capable of passing into one another by a process of conti- 
nuous change. A problem of far greater difficulty yet remains to be solved, the possible 
continuity of the liquid and solid states of matter. The fine discovery made some years 
ago by James Thomson, of the influence of pressure on the temperature at which lique- 
faction occurs, and verified experimentally by Sir W. Thomson, points, as it appears to 
me, to the direction this inquiry must take ; and in the case at least of those bodies 
which expand in liquefying, and whose melting-points are raised by pressure, the trans- 
ition may possibly be effected. But this must be a subject for future investigation ; 
and for the present I will not venture to go beyond the conclusion I have already drawn 
from direct experiment, that the gaseous and liquid forms of matter may be transformed 
into one another by a series of continuous and unbroken changes. 
4k 
MDCCCLXIX. 
