THE LIQUEFACTION AND SOLIDIFICATION OF AIR 
AND HYDROGEN. 
By W. N. HARTLEY, F.R.S.E., Kixg’s College, London. 
I N a former article in the u Popular Science Review ” (April 
1877), it was found necessary to give a sketch of the researches 
of Faraday and others on the liquefaction of gases ; and of 
Cagniard de la Tour and of Professor Andrews on the liquid 
and gaseous conditions of matter. According to the investi- 
gations of those observers a gas is nothing more than a highly 
attenuated vapour, and therefore capable of liquefaction. As, 
however, the hitherto possible deviations from the atmospheric 
temperature and pressure are insufficient to cause a change of 
state in many of these bodies, they have received the name of 
permanent gases. The distinction between a gas and a vapour 
is purely arbitrary. Ether in the gaseous state is called a vapour, 
while sulphurous acid is called' a gas. They are, however, both 
the vapours of liquids which boil at -f- 35° C. (95° F.) and 
— 10° C. (14° F) respectively. 
In a recent article in the “ Chemical News ” M. Raoul Pictet 
has very concisely stated the conditions to be fulfilled before we 
may hope to bring the molecules of a gas into such close contact 
with each other that they may cohere to form a liquid : — 
1 . The gas must be absolutely pure, with no trace of a foreign 
gas. 
2. A means of obtaining enormous pressures must be avail- 
able. 
3. A means of producing intense cold and subtracting heat at 
these low temperatures. 
4. The utilization of a large surface for condensation at these 
low temperatures. 
5. A means of utilizing the rapid expansion of the gas from 
extreme condensation to the atmospheric pressure — an expansion 
which, added to the preceding means, will compel liquefaction. 
Having fulfilled these five conditions, we may formulate the 
following alternative : — When a gas is compressed to 500 or 
