17 



The graduated and calibrated glass tube is filled with mercury, 

 and placed in the iron vessel containing this metal. The sub- 

 stance is contained in a thin bulb of known weight ; this is 

 then filled with liquid, again weighed, and then passed up to the 

 top of the divided tube. Surrounding this is a wide glass 

 cylinder open at both ends, and filled with water. The iron 

 vessel is now heated. The expansion due to the heat bursts the 

 bulb, and liberates the liquid, which is converted into vapour. 

 The water is agitated by a stirrer to keep the temperature 

 equally distributed. As soon as the temperature at which the 

 determination is to be made is reached the volume of the vapour, 

 the temperature of the water, and height of the barometer are 

 read off, the temperature of the air, and the difference in height of 

 the mercury inside and outside the tube, being ascertained at 

 the same time. 



From these data the vapour density may be calculated. 



94. Dumas'apparatus f or determining vapour densities. 

 Price IL 10s. 



1876. Exhibited by /. Orme fy Co. 



The method used with this apparatus consists in the 

 determination of the weight of a given volume of vapour. 



The bulb is first filled with dry air, and weighed, the tempera- 

 ture of the balance case being taken. A quantity of the liquid, 

 sufficient to produce enough vapour to expel the air, is then intro- 

 duced. The bulb is then heated in the iron vessel, filled with 

 water, oil, or paraffin according to the volatility of the sub- 

 stance, the temperature being raised to a point at least 30 to 

 50 above the boiling point of the liquid. The capillary tube is 

 sealed when no more vapour issues from it, and the temperature 

 noted. The globe is then removed, cleaned, and weighed when 

 cold. To determine the volume of the globe the point is broken 

 off under mercury, which rushes up and fills the globe ; the con- 

 tained mercury is weighed or poured into a graduated vessel to 

 obtain its volume. 



Having these data the vapour density may be calculated. 



95. Bunsen's apparatus for determining the specific 

 gravity of gases by effusion. 



5 E. 18. 1888. Made by C. Desaga, Heidelberg. 



The glass tube to contain the ga3 under examination is open 

 at the bottom, and furnished with a stopcock above. The 

 upper end of the tube is closed by a metal cap, which can be 

 screwed on and off. The lower end of this cap is closed by a 

 small piece of platinum foil sealed on to the cap, and pierced 

 with a small aperture. This small aperture is the only. exit for 



