Bug. 4, 1870] 
NATURE 
281 
spheres of pressure, stands nearly midway between the gas and 
the liquid ; and we have no valid grounds for assigning it to the 
one form of matter any more than to the other, The same observa- 
tion would apply with even greater force to the state in which car- 
bonic acid exists at higher tempeyatures and under greater pres- 
sures than those just mentioned. In the original experiment of 
Cagniard de la Tour, that distinguished physicist inferred that 
the liquid had disappeared, and had changed into a gas. A 
slight modification of the conditions of his experiment would 
have led him to the opposite conclusion, that what had been be- 
fore a gas was changed into a liquid. These conditions are, in 
short, the intermediate states which matter assumes in passing, 
without sudden change of volume, or abrupt evolution of heat, 
from the ordinary liquid to the ordinary gaseous state. 
“Tn the foregoing observations [ have avoided all reference to 
the molecular forces brought into play in these experiments. The 
resistance of liquids and gases to external pressure tending to 
produce a diminution of volume proves the existence of an in- 
ternal force of an expansive or resisting character. On the other 
hand, the sudden diminution of volume, without the application 
of additional pressure externally, which occurs when a gas is 
compressed, at any temperature below the critical point, to the 
volume at which liquefaction begins, can scarcely be explained 
without assuming that a molecular force of great attractive 
power comes here into operation, and overcomes the resistance 
to diminution of volume, which commonly requires the 
application of external force. When the passage from the 
gaseous to the liquid state is effected by the continuous pro- 
cess described in the foregoing pages, these molecular forces 
are so modified as to be unable at any stage of the process to 
overcome alone the resistance of the fluid to change of volume. 
““The properties described in this communication, as ex- 
hilited by carbonic acid, are not peculiar to it, but are generally 
true of all bodies which can be obtained as gases and liquids. 
Nitrous oxide, hydrochloric acid, ammonia, sulphuric ether, 
and sulphuret of carbon, all exhibited, at fixed pressures 
and temperatures, critical points, and rapid changes of volume 
with flickering movements, when the temperature or pressure 
was changed in the neighbourhood of those points. The critical 
pvints 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 
feunded on principles which are altogether arbitrary. Ether in 
the state of gas is called a vapour, while sulphurous 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 distinction 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 propertics 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 definition, 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 distin- 
guished by a surface of demarcation from the gas. If this defi- 
nition be accepted, carbonic acid will be a vapour below 31°, a 
gas above that temperature ; ether, a vapour below 200°, a gas 
above that temperature. 
“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 continuous change. A 
problem of far greater difficulty yet remains to be solved, the pos- 
sible contimuity of the liquid and solid states of matter. Lut this 
must be a subject for future investigation ; and fur 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.” 
JAmMEes THOMSON 
a 
NOTES 
Av last a sum of money has been voted for a new Natural 
History Museum. In introducing the vote the Chancellor of the 
Exchequer said the British Museum had long been suffering 
from repletion, and there were no means of exhibiting the 
valuable articles which, from time to time, were bought for the 
national collection. Five years ago the trustees resolved in 
favour of separating the collections, and it had been determined 
to separate the natural history department from the books and 
antiquities, or the natural history collection the typical mode 
of exhibition had been decided on, and the building required 
must cover at least four acres. Even the present collection 
would pretty well fill a building of these dimensions, and provi- 
sion must be made for further extension. The question was, 
where should this building be situated? and after referring to 
possible sites he referred to the locality which we were enabled 
to state some time ago had been chosen—a plot of ground 16% 
acres in extent, which the trustees of the Exhibition of 1851 sold 
to the Government at 7,000/. anacre. It therefore cost 120,000/., 
but is now worth 100,000/. more. The sale was coupled with 
the condition that any building erected upon the land must be 
for purposes of science and art. For seven years the land had 
remained waste, a sort of Potter’s field, and a scandal to that 
part of the metropolis. The Government now proposed to place 
on that piece of land the museum required for the natural history 
collection. It would occupy four acres ; there would be room 
for wings, and the outside estimate for the building was 350, 000/., 
not an unreasonable price, considering its extent. For the 
present, however, the Government merely asked for a small vote 
to enable them to clear the ground, and in order to take the 
opinion of the House. Railway communication had now made 
South Kensington easily accessible, and unless a more eligible, a 
more accessible, and a cheaper site could be suggested, he hoped 
the Committee would agree to the proposal, He might add 
that, if it were hereafter thought desirable to do so, there would 
be room enough on the same site for the Patent Museum, the 
necessity of which had been much insisted on. We trust that 
after the discussion which followed the introduction of the vote 
the scientific men will speak for themselves, and again let their 
wishes and opinions be heard. 
THe American Association for the. Advancement of Science 
met yesterday (Wednesday) at Troy. Professor W. Chauvenet 
is president for the year, 
Ir is gratifying to learn that some of the recommendations 
of the Royal Commission on Military Education, which were 
most inimical to the scientific instruction of the army, will 
not be carried out. 
By Imperial decree the Association Scientifique de France has 
been acknowledged to be an cfablissement d’utilité publique. 
THE French observers are making preparations for a com- 
bined attack on t!e 10th of August meteors. 
TuE list of pensions granted during the year ended the 2oth 
of June, 1870, and charged upon the civil list (presented pur- 
suant to Act 1 Victoria, cap. 2, sec. 6) has been published this 
week. Among them we note the following :—Mr. Augustus 
De Morgan, 100/., in consideration of his distinguished merits 
as a mathematician ; Mrs. Charlotte J. Thompson, 40/,, in con- 
sideration of the labours of her late husband, Mr. Thurston 
Thompson, as Official Photographer to the Science and Art 
Department, and of his personal services to the late Princ 
Consort ; Dame Henrietta Grace Baden Powell, 150/., in con- 
sideration of the valuable services to science rendered by her 
husband during the 33 years he held the Savilian Professorship 
of Geometry and Astronomy at Oxford; Miss Margaret 
Catherine Ffennell, Miss Elizabeth Mark Ffennell, and Mrs, 
Vv, 
