ON THE MEASUREMENT OF TEMPERATURE. 
on 
intervals, and round the cage so formed baize, 2 , was wound to form a plug a little 
smaller than the inner diameter of the tube, 0 . A glass tube, y, served as a 
continuation of the tube, -v, and the space between y and s was packed with animal 
wool. By means of this plug the vacuum vessel was effectually shielded from 
radiation from below. 
The method of tilling the thermometer bulb bas already been dealt with in a 
previous section of the work (p. 117). The ice point of the thermometer was of course 
determined before the tube, 0 , was placed in position. 
Before commencing an experiment with liquid hydrogen a vacuum vessel containii 
111 ' 
li(pud air was first introduced into the tube, 0 , and temporarily held in position by 
means of the plug; in this manner the bullj of the thermometer was cooled down 
to — 185° C. When the vessel containing the liquid air was again lowered, the wool 
was removed from the mouth of the vacuum vessel containing tlie hydrogen, which 
was brought below the apparatus, and slowly raised into position by means of the 
plug. The rubber sleeve, n, was then turned upwards over the tube, 0 , and secured 
in position by means of a piece of wire. 
I'lie actual measurement of the vapour pressure was made in the same manner as 
has been described in the case of oxygen (p. 138). One observer i-ead the pressures 
indicated by the manometer connected with the tul)e, m, in wliich pure hydrogen 
was liquefied, while the other took simultaneous readings of the main thermometei-, 
of the auxiliary thermometer, and of the temperature of the dead space. So steadily 
did the liydrogen boil under nmanal pressure that this operation could he carried out 
without the least difficulty. It is noticeable that the vapour pressure of the pure 
hydrogen in the bulb, m, was always slightly higher than the barometric pressure. 
This is to ])e attributed to tlie ])rese]ice (d‘ small (piantities of i)iq)urities dissolved in 
the main (piantity of hydrogen in the vacuum vessel, and confirms our reasons for 
ado})ting this method of measuring the va})Our pressures. 
In determining the vapour pressure of ll((uid hydrogen at lower jiressure we 
enqiloyed the following method : The tube, v, was connected by a short rubber tube 
to a Avide lead tube Avhich communicated Avith the exhaust puiu}), with a large glass 
glolie 300 millims. in diameter, with a mercury manometer, and Avith a fine adjust¬ 
ment sto})Cock, through Avhich air could be admitted to tlie apparatus and the 
pressure so regulated. 
The exhaust was maintained by means of a large tAvo-cylinder ‘Fleuss’ pum]», 
Avorked by a one horse-poAver electric motor. This pump, AA'hich Avas also used 
in connection AAuth the apparatus for producing liquid hydrogen, Avas arranged so 
that the Iavo cylinders could be worked in parallel to give a vacuum of 50 millims. of 
mercury, or in series to give a A’acuum of less than 1 millim. of mercury. For reasons 
AA’hich Avill lie entered into presently Ave haAm confined our measurements to pressure 
alioA'e 50 millims. of mercury. By opening and closing the fine adjustment cock, so 
as to admit more or less air into the appaiutus. the pressure could hi* maintained 
VOL. cc.—A. V 
