ON EVAPORATION AND DISSOCIATION. 
61 
the experimental tube is cooled, and ether rushes in to fill it. It is easy to make 
sure, by the absence of a bubble, of complete expulsion of air. The tube is then 
tilted, so that mercury covers the end of the experimental tube, and a portion of the 
latter is warmed. The ether boils off through the mercury, and, on cooling, its place 
is occupied by mercury. By tapping, the column of mercury may be made to descend 
to any desired point. When quite cold, the experimental tube is disconnected, placed 
in the iron cap, and gently warmed, so as to cause a globule of mercury to hang to 
its constricted open end. It is then plunged under the surface of the mercury in the 
branch of the iron tube, and the cap is screwed tight. From this description it will 
be noticed that all possibility of the presence of air in the liquid to be experimented 
on is completely excluded; and our results prove that this was the case, for the 
readings of vapour-pressure at different volumes of gas and liquid give, for the same 
temperature, absolutely identical results. 
Fig. 3. 
It was thus possible to alter volume by means of the screw; to read pressures 
accurately by the use of the high and low pressure gauges, the readings of which 
were compared when possible : and to secure constant known temperatures by means 
of the vapour-jacket. 
Experimental Results. 
1 . Vapour-pressures at Low Temperatures. 
Calibration of Thermometer. —The thermometer employed was a new one by 
Negretti and Zambra, divided into tenths of a degree, and registering from —20° 
to +50°. The zero-point at the atmospheric pressure was +0T9°. The bulb was 
immersed in mercury, contained in a vessel from which air could be exhausted by 
means of a Carre’s pump. Unless the bulb is dipped in mercury, the temperature it 
registers is altered by the cooling of the air by exhaustion, or heating by compression; 
the mercury serves to keep the temperature constant, It was found that the tempera- 
