Page:Mars - Lowell.djvu/103

 a colorless liquid, and then a solid of a floccular, snow-like character. It assumes, in short, under proper conditions of pressure and cold, the various appearances presented by water under higher temperatures, although it does so with very different degrees of ease. Superficially, therefore, the idea seems plausible. Let us see if it still seems so when critically examined.

Faraday made experiments on the relation of the congealing point of carbonic acid gas to the pressure, and found that at 0° C. it took a pressure of 36 atmospheres, that is, 540 pounds to the square inch, to solidify the gas, and that at −99° C., the lowest temperature with which he experimented, it took 1.14 atmospheres. At this point the curve representing the relation was becoming apparently asymptotic, that is, a slight decrease in pressure involved a great falling off of temperature. Under a pressure of one atmosphere, therefore, the temperature would be about −170° F., that is, on the surface of the Earth this would be the congealing point of the gas.

He found further that the curve for the liquefaction point lay very close to that for the congealing point, and approached yet closer as the pressure decreased. In other words, the gas passed almost immediately from the gaseous to the solid state.

In the light of these facts let us consider the