110 
PROFESSOR W. RAMSAY AND DR. S. YOUNG 
scale corresponded to 1° of temperature, it was only after several hours that the 
temperature became constant. When absolute constancy of temperature had been 
attained, the T-tube F was connected with the pump and with a gauge, and air was 
removed from the apparatus ; the diminution of pressure caused the bulb A to expand, 
and the mercury in the graduated stem to fall. This fall was found to be sensiblv 
equal for equal decrease of pressure ; and from the mean of a great number of readings 
a change of 9 - 04 millims. pressure was found to be equivalent to 1° of the scale ; hence 
To° corresponded to about 1 millim. 
A perfectly air-tight stopcock was then fused on to the T-tube F, and the apparatus 
was dried by exhausting the air with a Carre’s pump. The apparatus was then 
pumped empty of air with the mercury-pump, and the stopcock was closed. A 
reservoir, G, which had been sealed to the stopcock, was then filled with liquid nitric 
peroxide, and the stopcock was opened so as to admit two or three cubic centimetres of 
the liquid. The stopcock was then closed, and removed by sealing. 
As the height of the mercury in the graduated stem had been read before introducing 
the nitric peroxide, and as the introduction of the liquid caused a rise of the mercury 
corresponding to the pressure of its vapour, the apparatus acted as a barometer for 
temperatures below 0°. The vapour-pressure was regulated by the coldest part of the 
apparatus. The bulb E was placed in a well-stirred mixture of ice and hydrochloric 
acid, in contact with a thermometer, and the temperature and the position of the 
mercury in the graduated scale were read, with the following results :— 
