ON THE MEASUREMENT OF TEMPERATURE. 
157 
Thermometer 2. 
Thermometer 3. 
Vapour pressures . 
Temperatures .... 
764 • 4 millims., 30-40 millims. 
-2.53-37°C., -258-66°C. 
759'5 millims., 30-40 millims. 
- 250-35°C., - 255-67°C. 
As has already Ijeen pointed out (p. 156), the pressure on the gas in these thermo¬ 
meters was very small, and little weight can be attached to the results. 
2 . The Liquefaction of Hydrogen. 
The liquid hydrogen employed in these researches was obtained l)y means ot an 
apparatus devised and constructed by one of us twu) years ago. The apparatus as it 
was originally designed, has been described in the ‘Philosophical Magazine ’ (1901, 
vol. 17, 411), and such slight modifications as have since been introduced are 
referred to in the treatise, Travers on ‘ The Experimental Study of Gases,’ p. 206. 
The hydrogen was obtained by the action of dilute sulphuric acid upon commercial 
granulated zinc, contained in a cylindrical lead vessel, of which the joints had been 
soldered in the oxyhydrogen hlow-pipe. The gas was purified l)y passing it first 
through a solution of potassium permanganate; tlirough two glass towers, each 
1|- metres in length, containing broken pumice over which a sulphuric acid solution 
of potassium bichromate ran continuously ; tlirough a third glass tower containing 
pumice kept moist with silver nitrate solution ; and finally, through a wash bottle 
containing caustic potash solution, into the gasometer. In this way it was possible 
to obtain hydrogen from which lint little solid sejiarated during tlie process ot 
liquefaction.^'" 
In one experiment in which we failed to obtain liquid hydrogen, owing to the 
formation of solid air in the vacuum vessel enclosing the regeneratoi' coil, we found 
that the jiacking of the low-pressure piston of the compressor had become so worn 
that air was being taken into the cylinder with the hydrogen. With compressors 
of the “ Whitehead ” type, in which the piston-rod does not jiass through a gland, 
and the compression is only affected l:)y the inward motion of tlie }jiston, special care 
must be taken tliat the piston jjacking or cup-filire fits the cylindei'. 
In connection Avith the Avork on liquid hydrogen, Ave Avlsh to express (Aur thanks to 
Mr. Holding, Mechanic in the Chemical Department of University College, for his 
invaluable services. 
* In connection witli the preparation of the hydrogen, a curioirs oliservation \ra,s made Avhich has not 
been explained. The sih^er nitrate solution Avhich escaped at the bottom of tlie tower ajApeared almost 
immediately to deposit pure sih^er in the form of small liright metallic crystals. Very little sih^er 
appeared to form inside the toAver, and it seems as if some compound were produced which decomposed 
in contact with the air. The hydrogen had already passed through two towers containing acid chromate 
solution, and it is possible that hydrogen, like oxygen, liecomes more actiA'e when it takes part in a slow 
chemical reaction. 
