arsenicated Hydrogen Gas, 2&1 



siuch of tlie lye of caustic potash of a specific weight, in a di- 

 stilling apparatus fitted to collect the gas. At first only the air 

 of the vessels was disengaged ; nor was any gas collected until 

 the greater part of the water had evaporated and the lye had 

 thickened and begun to bubble up. A disengagement of gas 

 then commenced, which continued with rapidity until the mass 

 was dried. The gas which was obtained had no smell, and 

 burned with the slight and scarcely perceptible flame of hydrogen 

 gas. The residue in the retort occupied a large space ; when 

 withdrawn it was found to be spongy, of a deep red brown in the 

 inferior part, and inchning to black in the superior, in the ca- 

 vities of which some small octagonal crystals of arsenic were ob- 

 served. The mass quickly attracted the humidity of the air, and 

 its colour changed from a red to a blackish bro^vn. When water 

 was thrown over it, It dissolved quickly, became heated, and dis- 

 played a rapid ebullition, which however soon ceased. The gas 

 which developed itself iiad the smell of garlic, somewhat resem- 

 l)ling that of hydrosnlphvuic gas. After all the pieces of glass 

 were separated, and the finest of the crystals of arsenic washed, 

 the liquid part which was very alkaline was filtered, and the re- 

 sidue washed upon the filter. 



Obsei-vafions. 



1 have repeated the experiment of Gehlen, and have found that 

 it has been described with much exactness ; — I would only add 

 some remarks. 



The dissolution of potash ought to be perfectly caustic. It 

 is only, as Gehlen has observed, when it is very concentrated that 

 it acts upon the arsenic ; the gas, which then disengages itself 

 in abundance and with rapidity, is nothing else during all the 

 time of its disengagement but piue hydrogen gas ; it does not 

 deposit auv arsenic during its combustion, and consumes exactly- 

 half a volume of oxygen. 



When, instead of arsenic, its oxide is emjdoycd, it also give* 

 out a good deal of hydrogen, but only when tlie potash is nearly 

 desiccated and the temperature elevated. Tiie result of the de- 

 composition of water by the potash and the oxide of arsenic i» 

 an arseniate of potash, which may also be obtained by heating 

 ' together the oxide of arsenic and carbonate of potash fused and 

 deprived of water. In this case the carbonic acid is given out, 

 and one portion of the oxide is reduced to acidify the other. 



The mass of a deep red brown resulting from the action of the 

 caustic potash upon the arsenic appears to me to be a mixture 

 of arseniate and arseniurct of potash. It is liot likely that it 

 can consist of arsenite ; since, when the oxide of arsenic acts 

 lipon potash or upon its carbonitc, it produces an arseniate. 



The 



