Intelligence and Miscellaneous Articles. 71 
posed of materials derived from it. This has been traced on both 
sides of the road mentioned above for nearly two miles. For these 
and for other reasons given in the paper, the author is of opinion 
that, as he formerly maintained, there is a continuous upward suc- 
cession on the S.E. side of the road, from the quartz felsite at 
Brithdir to the Cambrian conglomerate on Bangor mountain. The 
district on the N.W. side of the road is so faulted that he can come 
to no satisfactory conclusions. The author is in favour of incorpo- 
rating the above-named quartz felsites with the overlying beds as 
one series, corresponding generally with the Pebidian of South 
"Wales — older than the Cambrian, though probably not separated 
from it by an immense interval of time. An analysis of the Brithdir 
quartz felsite by Mr. J. S. Teall was given, from which it ap- 
peared that the rock corresponds very closely with the " devitrified 
pitchstone " of Lea rock in the Wrekin district, described by !fr. 
Allport, but differs considerably in composition from those in the 
Ordovician rocks of North Wales. 
XII. Intelligence and Miscellaneous Articles. 
ON THE CEITICAL POINT OF LIQUEFIABLE GASES. 
BY J. JAMIN. 
4 TTEJN'TIOjN" having been called to the liquefaction of gases, I 
^*- proceed to lay before the Academy the reflections which the 
subject has suggested to me. The remarkable paper published by 
Andrews in 1870 * demonstrated that, in order to liquefy carbonic 
acid, pressures increasing up to 78 atmospheres must be employed 
when the temperature amounts to 31°. At that limit the phe- 
nomenon changes : certainly a rapid diminution of volume is still 
seen, and undulating and moving striae are observed as in a 
mixture of two liquids of different densities ; but there is no 
longer any liquefaction, however great may be the pressure ex- 
erted. In short, below 31° the gas is liquefiable ; above, it is so 
no longer. This is the critical point. 
These are incontestable facts ; and the notion of the critical 
point has rendered great services ; but it is unexplained, and 
perhaps inaccurately interpreted. I believe that gases are lique- 
fiable at any temperature when the pressure is sufficient, but that 
an unperceived circumstance has prevented the liquefaction being 
seen. To make this intelligible, let us take the experiments of 
Cagniard-Latour. 
A thick glass tube was half or two-thirds filled with water 
under the pressure of its vapour only ; it was hermetically sealed, 
and was then heated to 300° or 400°. According to known 
laws the quantity of vapour superposed to the liquid increases 
* Ann. de Chimit et de Physique, [4] xxi. p. 208. 
