LIQUID HYDROGEN. 261 



a screen (Fig. A, PI. II). The liquid hydrogen appears in oentle 

 ebullition and is perfectly clear, only there is a white solid deposit in 

 the bottom of the tube, which is really solid air. This may l)e shown 

 by removing- for an instant the cotton-wool stopper, when you see 

 a snow of solid air falling in the liquid. It is easy to arrange a 

 method of carrying liquid hydrogen in a small vacuum vessel in such 

 a way as to prevent the access of air. This is shown in fig. 1, PI. I, 

 where the vacuum vessel, after it has been filled by dipping it into the 

 main supply l)y means of a supporting wire, is surrounded with a glass 

 envelope, which becomes filled with an atmosphere of hydrogen gas 

 constantly maintained, thereb}' preventing the access of air. That 

 the density of the liquid is very small and is altogether unlike liquid 

 air is shown by dropping small pieces of cork, which float readily in 

 the latter liquid but sink instantly in the hydrogen (Fig. B, PI. II). 

 The real density of the liquid is only one-fourteenth that of water, so 

 that it is by far the lightest known liquid. This small density explains 

 the rapidity with Avhich the liquid is cleared on the entrance of the 

 air snow. The relative smallness of the gas bubbles produced in the 

 actively boiling liquid, which causes an appearance of opalescence, is 

 really due to the small surface tension of the liquid hydrogen. The 

 coefficient of expansion of liquid hydrogen is some five times greater 

 than that of liquid oxygen, and is comparable with that of carbonic 

 acid, about 5'^ from its critical point. The latent heat of evaporation 

 is about 190 units, and the specific heat of the liquid is very high, 

 and, so far as my experiments go, leads me to the value 6. This is in 

 very marked contrast to the specfic heat of liquid oxygen, which is 

 about 0.5. The extraordinary lowness of its boiling point is at once 

 apparent l)y cooling a piece of metal in the liquid and then removing 

 it into the air. when it will be seen to condense for a moment solid air 

 on its surface which soon melts and falls as a liquid air. This may be 

 collected in a small cup, and the production of oxygen demonstrated 

 by the ignition of a red-hot splinter of wood after the chief portion of 

 the nitrogen has evaporated. If a long piece of quill tubing sealed at 

 one end, but open at the other, is placed in the liquid, then the part 

 that is cooled rapidly fills with liquid air. On stopping any further 

 entrance of air by closing the end of the tube, the liquid air quickly 

 becomes solid, showing in the interior a hollow spindle from con- 

 traction, in passing from the liquid into the solid form (Fig. E, PI. II). 

 On })ringing the tube containing the solid from the liquid hydrogen 

 bath into the air we observe liquid air running from the surface while 

 the solid air inside is seen to melt (Fig. D, PI. II). Here is a tube 

 into which liquid oxygen has been poured. On placing it in liquid 

 hydrogen it freezes to a clear blue ice. Liquid nitrogen under simdar 

 circumstances forms a colorless ice. If instead of an open tube m 

 free air we employ a closed vessel of about a liter capacity to which 



