18 



the gas from the tube. The tube dips into a narrow mercury 

 reservoir, and is held in its place by a clamp sliding on a vertical 

 rod. 



In order that the gases should issue from the tube under 

 similar conditions of pressure, a float made as light as possible is 

 placed in the tube. This float carries at the top a black disc, 

 and lower down two black rings, serving as marks of level. 



The tube containing the gas and the float is placed in the 

 mercury so that the level of the mercury coincides with the 

 mark on the tube ; the float then is not visible through a tele- 

 scope directed at the mark. The stopcock is now opened, and 

 the gas allowed to escape through the aperture, the float rising 

 with the mercury. During this time the observer must watch 

 the level of the mercury through the telescope. Directly the 

 black disc appears observations of time must be made by a pen- 

 dulum beating half seconds. 



These observations are concluded directly the lower black 

 ring appears; the top of the float and the upper black ring 

 showing the approach of the two marks when observations have 

 to be made. 



By this means the time of effusion, of a column of gas of the 

 constant length between the black disc and the lower ring, is 

 obtained, issuing under pressures, the average value of which is 

 constant. 



The calculation of the specific gravity is based on the fact 

 that the specific gravities - of two gases, which stream out of a 

 fine opening in a thin plate, are very nearly proportional to the 

 squares of the times of effusion. 



Thus the squares of the times of effusion give the relation 

 between the specific gravities of the gases. 



96. Victor Meyer* vapour density apparatus. Price 

 5s. 



1886. Exhibited by Townson fy Mercer. 



The interior glass tube with enlarged end is heated by the 

 water or other liquid in the outer tube, until no more air passes 

 out of the delivery tube at the side, which is placed under water 

 in a pneumatic trough. The cork closing the tube is then 

 removed, and a weighed quantity of the substance, whose vapour 

 density is required, is dropped in, and the cork quickly, inserted. 

 As the substance volatilises it displaces air which passes out of 

 the delivery tube, and is collected in a graduated tube. From 

 the volume of the air displaced we get the volume of the vaporised 

 substance, and knowing its weight can calculate its vapour 

 density. 



