Page:Popular Science Monthly Volume 12.djvu/637

Rh open manometer on the flanks of a hill near the laboratory of Châtillon-sur-Seine.

N'  is a glass manometer which serves to check the readings of the mercurial apparatus.

This notable apparatus involves no danger, for the glass tube in which the gas is compressed presents a very small surface, and no serious result could follow were it to break. A few years ago an English physicist, Thomas Andrews, was led to infer that, for permanent gases, there exists a critical point of pressure and temperature, above which they cannot be brought to the liquid state. This opinion is confirmed by Cailletet's experiments. Each gas requires that a certain pressure be combined with a certain reduction of temperature: either the one or the other of these two conditions might be employed separately without any effect, even supposing them to reach a high intensity.

The first of the permanent gases liquefied by M. Cailletet was bioxide of nitrogen. As we have just said, unless the two conditions of compression and low temperature be united according to the critical points, the gas does not liquefy. Hence it is that bioxide of nitrogen has remained gaseous at a pressure of 270 atmospheres and a temperature of +8° Cent. Formene or marsh-gas liquefies at 180 atmospheres and +7° Cent.

"If," says M. Cailletet, "we inclose oxygen or pure carbonic oxide in the compression-apparatus; if we reduce these gases to a temperature of -29° Cent. by the aid of sulphurous acid at a pressure of about 300 atmospheres, both gases still retain their gaseous state. But if they be released suddenly, so, according to Poisson's formula, producing a temperature of at least 200° below the starting-point, we at once see a heavy mist, caused by the liquefaction or even, perhaps, the solidification of the oxygen or carbonic oxide. The