ON EUDIOMETERS. 281 



hydrogen with the remaining nitrogen. This was a mode 

 of analysing the air by the application of hydrogen, the 

 process invariably used in all accurate experiments at the 

 present time. 



Another substance used by Scheele was a solution of 

 liver of sulphur in water; that is obtained by fusing 

 together sulphur and what used to be called potash, but 

 which is really potassic carbonate. In this way he got a 

 mixture of potassic sulphides and potassic hyposulphite, 

 which is a mixture which has the power, especially when 

 in solution and slightly warmed, of absorbing oxygen ; and 

 Scheele employed an instrument for the absorption of 

 oxygen for this mixture, which was afterwards used by 

 Guyton de Morveau, who employed the potassic sulphide 

 at a high temperature placed in a retort with the neck 

 standing in water. The end of the retort was ground into 

 a long tube dipped into water. The retort was exactly 

 filled with atmospheric air, and the ground tube was 

 adjusted so as to prevent loss of gas by expansion in the 

 retort by the first application of heat. Of course when 

 heat is applied there is first expansion and then an absorp- 

 tion of oxygen, and the diminution of volume after cooling 

 is measured. 



Another great advance was made by Seguin, who used 

 phosphorus for the absorption of oxygen. A tube was 

 inverted over mercury, and into it was introduced a small 

 piece of phosphorus. By placing a red-hot iron near the 

 tube, sufficient heat radiated to melt the phosphorus against 

 the side of the vessel. Air was then introduced, and the 

 phosphorus being still warm was inflamed, and the oxygen 

 was removed. The gas was then poured out into another 

 graduated tube and measured. Berthollet also employed 

 phosphorus, but used it at the common temperature, and 

 the same process is still occasionally used. The objection 

 to phosphorus is that it has a certain vapour tension at 

 ordinary temperatures, and therefore T T ^th of the residual 

 volume must be deducted to allow for this quantity of 

 phosphorus vapour. By using phosphorus in the cold 

 state, six or eight hours are required for the complete 

 absorption of the oxygen. 



A great improvement was made in these processes by 

 Volta, who proposed to explode a mixture of air and 



