But before speaking of the combustibility and other chemical properties of hydrogen, we will first describe the physical properties of this gas, as we did in the case of water.
It is easy to show that it is one of the lightest gases.25
If passed into the bottom of a flask full of air, hydrogen will not remain in it, but, owing to its lightness, rapidly escapes and mixes with the atmosphere.
If, however, a cylinder whose orifice is turned downwards be filled with hydrogen, it will not escape, or, more correctly, it will only slowly mix with the atmosphere. This may be demonstrated by the fact that a lighted taper sets fire to the hydrogen at the orifice of the cylinder, and is itself extinguished inside the cylinder.
Hence, hydrogen, being itself combustible, does not support combustion.
The great lightness of hydrogen is taken advantage of for balloons. Ordinary coal gas, which is often also used for the same purpose, is only about twice as light as air, whilst hydrogen is 14½ times lighter than 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