THE n>M POSITION OF WATER, HYDROGEN 131 



O08957S gram ; that is, hydrogen is almost 1-U (more exactly, 14-43) 

 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 flows exceedingly rapidly from fine orifices, its molecules 

 (Chap. I.) being endued with the greatest velocity of movement. 24 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 less compressed than should follow from 

 the law of Mariotte,' 2 "' and with a rise of pressure it expands slightly 



data, for the temperature and moisture of the atmospheric strata in clear weather. All 

 the figures are given in the metrical system 1000 millimetres = 39'37 inches, 1000 kilo- 

 grams = '220 l-:',:',7"> 11 >s., 1000 cubic metres = 35316'5 cubic feet. The starting temperature 

 at the earth's surface is taken as = 15 C., its moisture 60 p.c., pressure 760 millimetres. 

 The pressures are taken as indicated by an aneroid barometer, assumed to be corrected 

 at the sea level and at long. 45. 



Although the figures of this table are calculated with every possible care from average 

 data, yet they can only be taken for an elementary judgment of the matter, for in every 

 separate case the conditions, both at the earth's surface and in the atmosphere, will differ 

 from those here taken. In calculating the height to which a balloon can ascend, it is 

 evident that the density of gas in relation to air must be known.' This density for 

 ordinary coal gas is from 0'6 to 0'35, and for hydrogen with its ordinary contents of 

 moisture and air from O'l to 0'15. 



Hence, for instance, it may be calculated that a balloon of 1000 cubic metres capacity 

 filled with pure hydrogen, and weighing (the envelope, tackle, people, and ballast) 727 

 kilograms, will ascend to a height of not much more than 4250 metres. 



24 If a cracked flask be filled with hydrogen and its neck immersed under water or 

 mercury, then the liquid will rise up into the flask, owing to the hydrogen passing 

 through the cracks about 3'8 times quicker than the air is able to pass through these 

 cracks into the flask. The same thing may be better seen if, instead of a flask, a tube 

 whose end is closed by a porous substance, such as graphite, unglazed earthenware, or a 

 gypsum plate, be employed. 



25 According to Boyle and Mariotte's law, for a given gas at a constant temperature the 

 volume decreases by as many times as the pressure increases; that is, this law requires 

 that the product of the volume v and the pressure p for a given gas should be a constant 

 quantity: pv C, a constant quantity which does not vary with a change of pres- 

 sure. In reality this equation does very nearly and exactly express the observed rela- 

 tion between the volume and pressure, but only within comparatively small variations 

 of pressure, density, and volume. If these variations be in any degree considerable, the 

 quantity /tv proves to be dependent on the pressure, and it either increases or diminishes 

 with an increase of pressure. In the former case the compressibility is less than it 

 should be according to Mariotte's law, in the latter case it is greater. We will call the 

 .tii -4 case a positive discrepancy (because then d (pv) VZ (p) is greater than zero), and the 

 second case a negative discrepancy (because then d (pv) /d (p) is less than zero). Deter- 

 minations made by myself, M. L. Kirpicheff, and Hemilian showed that all known gases 

 at low pressures, when considerably rarefied, present positive discrepancies. On the 

 other hand it appears from the researches of Cailletet, Natterer, and Amagat that all 

 gases under great pressures (when the volume obtained is 500-1000 times less than 

 iimler the atmospheric pressure) also present positive discrepancies. Thus under a pres- 

 >nre of 2700 atmospheres air is compressed, not 2700 times, but only 800, and hydrogen 

 1000 times. Hence the positive kind of discrepancy is, so to say, normal to gases. And 

 this is easily understood. Did a gas follow Mariofcte's law, or were it compressed to a 



K 2 



