Additional Notes. 421 



Note (X), p, 133. — The numerous eudiometers proposed by 

 different chemists may be reduced to five. 



1. The first is that invented by Dr. Priestley, in wluJi /«i- 

 trous gas is mixed, over water, with the air tlie purity of whicJi 

 it is wished to ascertain. The diminution of the volume of this 

 mixture is proportioned to the quantity of oxygen contained in 

 the air, which is rapidly absorbed by the nitrous gas, and tlie 

 nitric acid thus formed is also rapidly absorbed by the water. 

 This eudiometer has received various modifications and improve- 

 ments by Falconer, Cavendish, and von Humboldt, but is 

 still liable to considerable anomaly and inaccuracy in its indi- 

 cations. 



2. The second kind of eudiometer was proposed by Volta. 

 His method was to mix given proportions of the air to be exa- 

 mined, and hydrogen gas, in a graduated glass tubej to tire the 

 mixture by an electric spark 3 and to judge of the purity of the 

 air by the bulk of the residuum. But this furnishes a measure 

 eren less to be depended on than the preceding. 



3. Scheele was the inventor of tlie third kind of eudiometer. 

 It is merely a graduated glass vessel, containing a given quan- 

 tity of air, exposed to a newly-prepared liquid alkaline or earthy 

 sulphuret, or to a mixture of iron filings and sulphur, formed 

 Into a paste with water. These substances absorb the whole of 

 the oxygen of the air, which converts a portion of the sulphur 

 into an acid. The oxygen contained in the air thus examined is 

 judged of by tlie diminution of bulk which tlie air has under- 

 gone. This method is simple, and as accurate as any otlier. 

 The only objection to which it is liable is the slowness. of the 

 process. But this objection has been removed by M. de Marti, 

 who has brought the eudiometer of Scheele to a great degree of 

 accuracy, by improving the apparatus, and, instead of iron 

 filings and sulphur, using the hj/droi^niated sulpliiircts only. 



Guyton-Morveau employs sulphuret of poUii,h, and measures 

 the proportion of oxygen present by the quantity absorbed by 

 the sulphuret. 



4. In the fourth kind of eudiometer, the abstraction of tl.e 

 oxygen of air is accomplished by means of phosphorus. This 

 eudiometer was first proposed by Achard. It was considerably 

 improved by Reboul, Seguin, Lavoisier, and, above all, by 

 Berthollel, who has rendered it equal in simplicity with the 

 eudiometer of de Marti, and scarcely inferior to it i.'i pre- 

 cision. 



