them, and they will be mixed and confounded.  They will no longer be distinguishable by their heat of constitution.  It is true that, in passing into the state of a vapor, a liquid absorbs a great deal of latent heat, but that is employed in scattering the molecules and keeping them at a distance; and there will be none of it if the distance does not increase.  We are then, at this stage of our experiments, in the presence of a critical point, at which we do not know whether the matter is liquid or gaseous; for, in either condition, it has the same density, the same heat of constitution, and the same properties.  It is a new state, the gaso-liquid state.  An experiment of Cagniard-Latour re-enforced this explanation of the phenomena.  Heating ether in closed vessels to high temperatures, he brought it to a point where the liquid could be made wholly to disappear, or to be suddenly reformed on the slightest elevation or the slightest depression of temperature accordingly as it was raised just above or cooled to just below the critical point.  The discovery of these properties suggested an explanation of the failure of previous attempts to liquefy air.  Air at ordinary low temperatures is in the gaso-liquid condition, and its liquefaction is not possible except when a difference exists between the density of the vapor and that of the liquid greater than it is possible to produce under any conditions that can exist then.  It was necessary to reduce the temperature to below the critical point; and it was by adopting this course that MM.  Cailletet and Raoul Pictet achieved their success.  The rapid escape of the compressed gas itself from a condition of great condensation at an extremely low temperature was employed as the agent for producing a greater degree of cold than it had been possible before to obtain.  M. Cailletet used oxygen escaping at -29 deg.  C. from a pressure of three hundred atmospheres; M. Raoul Pictet, the same gas escaping at -140 deg. from a pressure of three hundred and twenty atmospheres; and both obtained oxygen and nitrogen, and M. Pictet hydrogen, in what they thought was a liquid, and possibly even in a solid form.

Still, it could not be asserted that hydrogen and the elements of the air had been completely liquefied.  These gases had not yet been seen collected in the static condition at the bottom of a tube and separated from their vapors by the clearly defined concave surface which is called a meniscus. The experiments had, however, proved that liquefaction is possible at a temperature of below -120 deg.  C. (-184 deg.  Fahr.).  To make the process practicable, it was only necessary to find sufficiently powerful refrigerants; and these were looked for among gases that had proved more refractory than carbonic acid and protoxide of nitrogen.  M. Cailletet selected ethylene, a hydrocarbon of the same composition as illuminating gas, which, when liquefied by the aid of carbonic acid and a pressure of thirty-six atmospheres, boils at -103 deg.