MINERAL CAVITIES AND THEIR CONTENTS. 
123 
acid, were screwed on to a reservoir of mercury. Compression 
of the gas within the tubes was accomplished by the action of 
a screw upon the mercury; the joints of the apparatus, being 
rendered tight by greased leather washers and cobbler’s wax, were 
capable of sustaining a pressure equal to 500 atmospheres for days 
together. At no temperature above 30 o, 92 C. (87°*6 F.) was it 
found possible to maintain the liquid condition of carbonic acid, 
no matter what the pressure might be. This temperature is called 
by Professor Andrews the u Critical Point ” of carbonic acid, and 
the condition of the substance is that mentioned by Faraday as 
“ Cagniard de la Tour’s state.” Each gas and liquid has a fixed 
and definite temperature for its Critical Point, and this, in such 
gases as hydrogen, nitrogen, and oxygen, is below the reach of 
Fig. 1, Fig. 2. 
x 46 diameters. 
Fig. 1 . — a is gaseous carbonic acid; b is liquid carbonic acid ; c is water. These 
letters also apply to the smaller cavities. 
Fig. 2. — The specimen warmed to 31° C. ; the line of demarcation between gas 
and liquid has disappeared. The water is seen undisturbed. 
any refrigerative apparatus at present known to us ; hence any 
pressure we may put upon them will fail to cause liquefaction. 
Professor Andrews last summer lectured to the Fellows of 
the Chemical Society on the methods of research which are 
peculiarly his own. He completely charmed his audience by his 
easy conversational manner and by the way he took one into his 
confidence, explaining all his difficulties and how he overcame 
them. Anything of the kind more delightful has seldom been 
heard. Every one shared his enthusiasm. 
1 Figs. 1, 2, 3, 4, 8, 9 and 10 are reproduced from the a Journal of the 
Chemical Society,” by permission of the Council. 
