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VERY one knows that the matter which constitutes the various natural bodies occurs in three different forms, namely, the solid, the liquid, and the gaseous states. So, too, every one knows that the state of a body is not at all immutable: a solid may be fused and volatilized; a liquid may become a solid, or be transformed into vapor; a gas may be changed into a liquid or a solid—all these changes occurring according to the conditions of temperature or of pressure to which the solids, liquids, or gases, are subjected. Water turns into ice under the action of cold; into steam under the action of heat. Sulphur, phosphorus, the metals, and most solid bodies, may in like manner assume these three states. Chlorine, protoxide of nitrogen, carbonic acid, etc., may be liquefied or solidified. To this end we have only to bring the molecules nearer to one another by compressing them, or subjecting them to the action of cold.

Faraday succeeded in liquefying a certain number of gases by compression and refrigeration, but there still remained a number that proved absolutely refractory to the most powerful agencies; hence these gases were called permanent. They are hydrogen, nitrogen, oxygen, carbonic oxide, bioxide of nitrogen, and formene (marsh-gas). Chemists, it is true, were quite confident that these gases, like all others, were subject to the general laws of bodies; it was held to be certain that the gases just named would, like the others, yield to sufficiently high pressure or refrigeration. But, nevertheless, they still remained bodies sui generis, defying, so to speak, the powers of the chemist, and their change of state presented itself as a weighty problem, the solution of which was all the more alluring in proportion to the difficulties with which it was surrounded. Berthelot, as we know, subjected them to the enormous pressure of 800 atmospheres, and to a refrigeration of more than 100° below zero, Centigrade; but all was in vain, and the permanent gases justified their name.

This is so no longer. A retired manufacturer, who at the same time is a distinguished man of science, M. Cailletet, has subdued the permanent gases, having succeeded in liquefying and solidifying them. This result, which is one of the most interesting achievements of our time, must unquestionably be regarded as a new and a grand conquest of matter by science.

Nearly at the same moment, another ingenious investigator and inventor, M. Raoul Pictet, reached the same result with regard to oxygen gas. We will pass in review successively the experiments of