CHAPTER II THE COMPOSITION OF WATER, HYDROGEN

air. A very simple experiment with soap bubbles very well illustrates the application of hydrogen for filling balloons. Charles, of Paris, showed the lightness of hydrogen in this way, and constructed a balloon filled with hydrogen almost simultaneously with Montgolfier. One litre of pure and dry hydrogen²⁶ at 0° and 760 mm. pressure weighs 0·08986 gram; that is, hydrogen is almost 14½ (more exactly, 14·39) times lighter than air. It is the lightest of all gases. The small density of hydrogen determines many remarkable properties which it shows; thus, hydrogen passes exceedingly rapidly through fine orifices, its molecules (Chapter I.) being endued with the greatest velocity.²⁷ At pressures somewhat higher than the atmospheric pressure, all other gases exhibit a greater compressibility and co-efficient of expansion than they should according to the laws of Mariotte and Gay-Lussac; whilst hydrogen, on the contrary, is compressed to a less degree than it should be from the law of Mariotte,²⁸ and with a rise of pressure it expands slightly less than at the atmospheric pressure.²⁹ However, hydrogen, like air and many other gases which are permanent at the ordinary temperature, does not pass into a liquid state under a very considerable pressure,³⁰ but is compressed into a lesser volume than would follow from Mariotte's law.³¹ From this it may be concluded that the absolute boiling point of hydrogen, and of gases resembling it,³² lies very much below the ordinary temperature; that is, that the liquefaction of this gas is only possible at low temperatures, and under great pressures.³³ This conclusion was verified (1877) by the experiments of Pictet and Cailletet.³⁴ They compressed gases at a very low temperature, and then allowed them to expand, either by directly decreasing the pressure or by allowing them to escape into the air, by which means the temperature fell still lower, and then, just as steam when rapidly rarefied³⁵ deposits liquid water in the form of a fog, hydrogen in expanding forms a fog, thus indicating its passage into a liquid state. But as yet it has been impossible to preserve this liquid, even for a short time, to determine its properties, notwithstanding the employment of a temperature of -200° and a pressure of 200 atmospheres,³⁶ although by these means the gases of the atmosphere may be kept in a liquid state for a long time. This is due to the fact that the absolute boiling point of hydrogen lies lower than that of all other known gases, which also depends on the extreme lightness of hydrogen.³⁷