302 EESEAUCHES ON EVAPORATION AND DTSSOOIATION. 



1)oils or volatilizes at audi a temporaturo that its vapour 

 exerts a pressure just equal to the external pressure. 



A modiiied Wollaston's cryophorus was used, with ther- 

 mometers dipping into each bulb. Water which had been 

 boiling for some time was placed in the bulbs, and boiled 

 down, so that the steam expelled the air very completely 

 from the apparatus. The opening through which the steam 

 escaped was then closed. By suitable manipulation the 

 bulb of one thermometer was coated with ice, not in contact 

 with the sides of the outer bulb. When this bulb was 

 surrounded by a hot bath, the other bulb being placed in 

 a freezing mixture, the two thermometers registered the 

 same temperature, but on admitting a minute quantity of 

 air, the thermometer coated with ice showed a higher 

 temperature than the thermometer in the condenser, by an 

 amount which could be approximately calculated from the 

 vapour-pressures of ico determined by liegnault, the air 

 admitted adding its pressure to the va])our-pressure of the 

 ice in contact with the sides of the condenser. Similar 

 results were obtained with benzene and witli acetic acid. 

 Determinations of the vapour-pressures of camphor by both 

 statical and dynamical methods were found to be completely 

 concordant. Tor the statical determinations, the camphor 

 was placed in a barometer-tube, with special precautions to 

 exclude air, and the tube was jacketed with the vapours 

 of pure liquids boiling under atmospheric. ])ressure. In 

 the accompanying figure, which represents the dynamical 

 method, A represents the block of camphor round the 

 thermometer B, inserted through an indiarul)ber cork into 

 the tube C, jacketed by the wider tube 1). C is connected 

 with the condenser E, from which a tube F passes to the 

 Sprengel's pump. Air could be admitted through a l)ranch 

 G, closed by an indiarubbor tube and a screw-clip. 



