548 
DR. A. SCOTT ON THE COMPOSITION OE WATER BY VOLUME. 
both those of the barometer EE, as well as those in C and D, by means of a catheto- 
meter Z (fig. 4) constructed specially for the purpose. This consisted essentially of a 
vertical steel axis carrying two horizontal telescopes wfith cross wires, which were set 
one at each level, and then the axis rotated to the scale F, and the heights registered 
to 0 millim. This method was necessitated from the want of steadiness in the floor 
of the room in Durham School Laboratory, where the majority of the experiments 
were carried out. The same arrangement (which is very convenient as well as 
accurate) was also used in the later experiments which were carried out under more 
favourable conditions in the gas analysis room of the Cambridge University Chemical 
Laboratory. 
Fig. 2 shows the earlier form of measuring vessel used wdien the varying pressures 
and volumes were both measured. The tubes B, C, and D were all of the same 
diameter, so that no capillarity corrections were required. 
Fig. 3. 
S, Flask for sodium. 
T, Washing tube, with very dihite hydrochloric acid. 
U, Exit tube for hydi’ogen, sealed to U (fig. I) Avheu in use. 
W, Small india-rubber stopper. 
Fig. 3 shows the simple apparatus employed for the preparation of hydrogen from 
sodium by acting on it with steam. When it was used the end of the tube U was 
sealed to the tube U (fig. l), which contained palladium in its wider part, P. The 
only india-rubl)er connection is at W, where there is a stopper of that material, but 
diftusion through this was practically impossible to any injurious extent, as the space 
between the tubes was very narrow and the stopper was 6 to 8 milliras. long, and 
after setting up the apparatus it was sealed over with parafilu containing enough 
vaseline to prevent its cracking, whilst it allowed of its being easily shaken. 
