130 



PRINCIPLES OF CHEMISTRY 



hydrogen will not remain in it, but, owing to its lightness, rapidly 

 escapes and mixes with the atmosphere. If, however, a cylinder whoso 

 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 advan- 

 tage 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 

 I 4^ 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 con- 

 structed a balloon filled with hydrogen almost simultaneously with Mont- 

 golfier. One litre of hydrogen 23 at and 760 mm. pressure weighs 



- 3 The density of hydrogen in relation to the air has been determined by accurate 

 experiments. The first determination, made by Lavoisier, was not entirely exact ; taking 

 the density of air as unity, he obtained 0'0769 for that of hydrogen that is, hydrogen as 

 thirteen times lighter than air. Later determinations have corrected this figure, the 

 most accurate determinations being due to Thomsen, who obtained the figure 0*0698 ; 

 Berzelius and Dulong, who obtained 0'0688 ; and Dumas and Bunseii. who obtained 

 0-06945. But the most exact determination of all is, without doubt, due to Regnault. 

 He took two spheres of considerable capacity, which cm en hied equal volumes of air 

 (thus avoiding the necessity of any correction for weighing them in air). Both spheres 

 were attached to the scale pans of a balance. One was sealed up, and the other first 

 weighed empty and then full of hydrogen. Thus, knowing the weight of the hydrogen 

 filling the sphere, and the capacity of the sphere, it was easy to rind the weight of a litre 

 of hydrogen ; and, knowing the weight of a litre of air at the same temperature and 

 pressure, it was easy to calculate the density of hydrogen. Regnault, by these experi- 

 ments, found the average density of hydrogen to be 0'06926 in relation to air, or including 

 the necessary corrections 0'06949. 



In this book I shall always refer the densities of all gases to hydrogen, and not 

 to air ; therefore, for the sake of clearness, I will cite the weight of a litre of dry pure 

 hydrogen in grams at a temperature t and under a pressure H (measured in millimetres 

 of mercury at 0, in long. 45). The weight of a litre of hydrogen 



1 



= 0-08958 x x __ 

 760 1 - 



gram. 

 I) -008(57* 



For aeronauts it is very useful to know, besides this, the weight of the air at different 

 heights, and I therefore insert the adjoining table, constructed on the basis of Glaisher's 



