•392 Scipntific Proceedings, Eoyal Duhlin Society. 



surrounded by vapour quite free from air. Under these conditions the 

 liquid very quickly reaches its true boiling-point under the pressure 

 registered by the gauge; but it cannot become superheated, since evaporation 

 takes place with perfect freedom, and, of course, ebullition is impossible. 



The temperature and pressure are read ; and a small quantity of air is 

 then admitted to raise the pressure, when the temperature rises at once, and 

 soon becomes constant again at the boiling-point under the higher pressure. 

 Fresh quantities of liquid are added in small quantities, as required, from 

 the reservoir. A large number of readings can thus be taken in an hour or 

 two. The pressure may, if desired, be reduced by means of the pump, 

 when the temperature falls ; and the reduction of pressure can be carried on 

 until the boiling-point of the liquid falls to the temperature of Ihe condenser. 



2. For pressures from about 500 mm. to that of the atmosphere, a 

 long-necked distillation-bulb with reflux condenser was employed, the 

 tliermometer being placed in the vapour. 



A method similar in principle was employed by Regnault for the 

 determination of boiling-points under pressures higher than one atmosphere, 

 closed iron vessels being substituted for the ordinary glass apparatus. 



Errors due to "bumping" or superheating of the vapour when the quantity 

 of liquid is very small may be almost completely obviated by covering the 

 bulb of the thermometer with a very little cotton- wool, so as to keep it 

 always moist. The whole of the mercury, not only in the bulb, but also in 

 the stem of the thermometer, should be heated by the vapour, so as to avoid 

 the somewhat uncertain correction for the cool portion. 



3. For pressures higher than that of the atmosphere an apparatus was 

 employed which is similar in principle to that devised by Andrews for his 

 classical researches on carbon dioxide. 



The apparatus, which was also employed for the determination of the 

 specific volumes of liquid and saturated vapour, as well as for the 

 compressibilities of liquid and unsaturated vapour, is shown in fig. 3. It 

 consists of a long wrought-iron tube, with three shorter tubes at right angles 

 to it. The long tube is securely fixed in a horizontal position, the branches 

 being vertical. One eud of the horizontal tube is closed ; the other end is 

 fitted with a thick steel ring. A, and perforated screw, B, through both of 

 which passes a smooth steel plunger, C. The space between the ring and 

 the screw serves as a stuffing-box to prevent leakage. It was filled with 

 the wick of a common tallow candle from which most of the outer part of 

 the tallow had been removed, and which was wound round the plunger 

 before the screw was inserted. The outer half of the plunger is cut as a 

 screw, and passes through the nut, D, which is rigidly fixed to the same 



