247 



fourth the density according to the formula and tlie fifth the difference 

 of the latter two. 



In addition to the liquid-densities the density of solid hydrogen 

 was also determined. It was first of aW ascerisiineó theii solid /iychvge7i 

 is /wavier than liquid Jujdrogen and, therefore, that on freezing 

 contraction takes jjlace. For this purpose the hydrogen in the cryostat 

 was frozen; a solid crust is then formed on the liquid. If the pressure 

 is now made to rise slowly, this crust divides into fragments which 

 sink down in the liquid and collect at the bottom of the vacuum- 

 glass. It was not till this observation was made, that we ventured 

 to freeze the hydrogen in the dilatometer without fear of bursting 

 the very carefully calibrated instrument. 



In order to be sure, that the freezing would proceed from the 

 bottom, the tube of the dilatometer was movable in a stuffing box, 

 which was fitted in the lid of the cryostat. After the hydrogen in 

 the cryostat had been frozen to an opaque solid mass, the dilatometer 

 was pushed down slowly. The hydrogen in the dilatometer was then 

 frozen lo a homogeneous transparent mass. At the moment of 

 reading the volume of the solid hydrogen, the pressure in the cryostat 

 was 0.66 cm. 



In order to obtain an estimate of the temperature, the vapour 

 pressures of solid hydrogen being practically unknown, the vapour 

 pressures according to Kamerlingh Onnes and Keesom were extra- 



1 



polated by means of the log p, — diagram, in which the vapour 



pressure curves are known often to be very nearly straight lines, 

 neglecting the break in the curve at the triple point. In that manner 

 the temperature corresponding to a pressure of 0.66 cm. was 

 found to be 



— 262°.0. 

 The density or solid hydrogen at this temperature was found to be 



^rsoi = 0.08077 . 



Putting the triple point temperature, according to the same measu- 

 rements at 



^cocx. lic^. vap. &ul. ^— nOo .li 



extrapolation by means of the above formula gives for the liquid 

 density at that point 



('Fcoex.liq.vap.sol. — — ^^.^7/ i\Jv. 



Assuming that the density of solid hydrogen at the triple point 

 does not differ appreciably from that at — 262^, the contraction on 

 freezing is found to be about 4.8 7o of the liquid volume. 



