208 SCIENTIFIC APPARATUS. 



densities, an unknown quantity of the volatile body is introduced 

 into a weighed flask of known capacity, the neck of which is 

 drawn out to a fine point. The flask is plunged into a vessel con- 

 taining a hot liquid, and when vapours cease to issue the point 

 is sealed with a blow-pipe flame. After cooling, the flask is 

 weighed, whereby the quantity of substance in the gaseous con- 

 dition at the moment of closing the vessel is determined. This 

 method does not give such accurate results as that of Gay-Lussac 

 unless the substance is absolutely pure. 



Deville and Troost, by replacing the glass flask by one of porce- 

 lain, which was heated in the vapour of mercury, sulphur, cad- 

 mium, or zinc, have succeeded in determining the specific gravities 

 of the vapours of several substances of high boiling points, and 

 have thus obtained results of great value. 



In 1873, Dewar and Dittmar, by the use of an iron bottle 

 determined approximately the vapour-density of potassium. 



ANALYSIS OF GASES. 



The analysis of gases has recently attracted considerable atten- 

 tion, and has been brought to a condition of great exactness. 

 The object of the first gas analysts was to determine the salubrity 

 of the air, which was thought to depend on the varying proportions 

 of the constituent gases in different localities. The process was 

 hence called eudiometry, or measurement of the purity of air. This 

 was first attempted by Dr. Hales, and the processes are described 

 in his book on Vegetable Staticks published in 1727. Fontana in 

 1770, Priestley in 1 7 74, and Cavendish a little later, employed nitrous 

 air (nitric oxide, as it is now called) for removing the oxygen from 

 a measured volume of air, Scheele employed a solution of hepar 

 sulphuris, and Guy ton de Morveau a piece of solid sulphide of 

 potassium, heated in a retort containing the air. Seguin used 

 warm phosphorus, and Berthollet phosphorus at the ordinary tem- 

 perature for the same purpose. Volta first introduced the explosion 



