1899.] Centenary Commemoration, 1799-1899. 215 



any vessel containing air, when it will be seen the vacuum produced by- 

 hydrogen cooling is equal to that of a Torricellian vacuum (Plate II.). 

 To reach such a high exhaustion the solid oxygen and nitrogen, at the 

 boiling point of hydrogen must be practically non- volatile or have an 

 exceedingly small vapour pressure. If the ordinary air contains free 

 hydrogen, helium, &c. which are non-condensable in this way of work- 

 ing, then the vacuum would not be so high as with pure oxygen or 

 nitrogen. This method may be used to separate the incondensable 

 gases from the air. Such air vacua when examined spectroscopi- 

 cally show the lines of hydrogen, helium and neon. We may now 

 employ this process to produce high vacua, and test their exhaustion 

 by the character of the electric discharge. Vacuum tubes which 

 have been prepared in this way show extraordinary resistance to the 

 passage of the electric discharge ; they also show the marked phosphor- 

 escence of the glass, characteristic of Crookes tubes (Figures F and 

 G, Plate III.). It is, however, the rapidity with which such high ex- 

 haustions can be attained that is so interesting. You will observe 

 that this large Geissler tube, previously exhausted to some three 

 inches pressure, will, when the end part is immersed in liquid hydro- 

 gen, pass through all the well-known changes in the phases of 

 striation : the glow on the poles ; the phosphorescence of the glass ; 

 in the space of a fraction of a minute. From this it follows that 

 theoretically we need not exhaust the air out of our double-walled 

 vessel when liquid hydrogen has to be stored or collected. This makes 

 a striking contrast to the behaviour of liquid air under similar cir- 

 cumstances. The rapid exhaustion caused by the solidification of the 

 air on the surface of a double-walled unexhausted test-tube, when 

 liquid hydrogen is placed in it, may be shown in another way. Leave 

 a little mercury in the vessel containing air, just as if it had been 

 left from making a mercurial vacuum. Now we know mercury, in 

 such a vacuum, can easily be made to distil at the ordinary temperature 

 when we cool a part of the vessel with liquid air, so that we should 

 expect the mercury in the unexhausted test-tube to distil on to the 

 surface cooled with the liquid hydrogen. This actually takes place. 

 A rough comparison of the relative temperatures of boiling hydrogen 

 and oxygen may be made by placing two, nearly identical, hydrogen 

 gas thermometers operating at constant pressure side by side and 

 cooling each with one of the liquids (Plate IV.). It will be seen 

 that the contraction in the thermometer cooled with liquid hydrogen 

 elevates the liquid some six times higher than that of the correspond- 

 ing liquid column of the thermometer placed in the liquid oxygen. 

 A constant volume hydrogen thermometer constructed as shown in 

 Plate V. gave the boiling point of 21° absolute or —252° C, and a 

 similar helium thermometer gave the same result. The critical tem- 

 perature is about 32° absolute or — 241° C, and the critical pressure 

 about 15 atmospheres. If a closed vessel is full of hydrogen gas at 

 atmospheric pressure, then, unlike the air vessels, it shows no conden- 

 sation when a part of it is cooled in liquid hydrogen. To produce 



