CHEMICAL APPARATUS AND PRODUCTS. 207 



as the square roots of their specific gravities. The method 

 consists in observing the time required for the passage of a 

 measured volume of gas through a very fine hole in a platinum 

 plate. 



When the specific gravity of the vapour of a volatile solid or 

 liquid is to be determined, the method of Gay-Lussac is usually 

 employed. This consists in introducing a small glass bulb con- 

 taining a weighed quantity of the substance into a graduated tube 

 inverted over mercury in an iron trough. The tube is then sur- 

 rounded by a glass cylinder^open at both ends, and with the lower 

 edge depressed in the mercury in the trough. Water or oil is 

 placed in the cylinder, and heat applied to the bottom of the 

 mercurial trough. When the apparatus has been uniformly heated 

 to the proper temperature, the volume of the vapour is read off, 

 and by the usual methods of calculation the vapour density is 

 obtained. 



Instead of using the iron mercurial trough, it is more conve- 

 nient to introduce the tube supported by an iron stirrup with a 

 mercury cup at the bottom into a large test tube containing water 

 or melted paraffin. In this way the escape of mercurial vapour is 

 avoided. 



A modification of this process has recently (1868) been made 

 by Hofmann. He employs a graduated tube of a metre in length, 

 which is filled with mercury and inverted over a mercurial trough. 

 A known quantity of the substance enclosed in a very small 

 stoppered bottle is passed up into the tube, the upper part oi 

 which vessel is surrounded by a wide glass tube, through which 

 a current of the vapour of a volatile liquid, such as alcohol, 

 water, or aniline is passed. The volume of the vapour thus pro- 

 duced is read off in the ordinary way. This method has the 

 advantage of causing the volatilisation to take place at a lower 

 temperature than would otherwise be necessary in consequence of 

 the diminished pressure produced by the column of mercury. 



In the process of -Dumas for the determination of vapour 



