458 MM. Troost and Deville on Vapour Densities. 



precipitating metallic gold and palladium from their solutions, 

 and forming with silver solution silicate of the suboxide of silver. 

 It also reduces copper salts ; and from selenious, telluric, and 

 sulphurous acids it liberates selenium, tellurium, and sulphur. 



The analyses lead to the formula Si^ 0'', 2H0 ; and the sub- 

 stance can have no other composition, if the protochloride have 

 the formula Si^CP, 2HC1. But this is still an open question. 

 Buff and Wohler have reason to believe that there is a still lower 

 chloride and a corresponding oxide, but all their endeavours to 

 decide this point were fruitless. They hope to resume it when 

 they possess more silicon. 



It was an old observation of Scliaf heutl, that the residue from 

 the solution of cast iron in acid, after being well washed out, 

 liberated hydrogen when treated with ammonia. The discovery 

 of the new oxide of silicon rendered it probable that this residue 

 contains the hydrated oxide of silicon. A recent investigation 

 by Wohler* showed that this is the case; and it seems there- 

 fore that siliciuret of iron, when dissolved in acid, does not form 

 silica, but this new hydrate, — a deportment shared by siliciuret 

 of manganese, as has been noticed by Wohler. 



Dumas' method of taking vapour densities, so very accurate 

 and easy of execution, is admirably fitted for all substances of 

 moderately low boiling-points. But many chemical substances, 

 and especially inoi'ganic compounds, volatilize at such high tem- 

 peratures that an oil-bath cannot be used, and even glass vessels 

 soften. In any general method for the determination of the 

 vapour densities of substances with high boiling-points, two 

 questions demand attention : first, the manner in which the heat 

 is to be applied ; and second, the nature of the vessels in which 

 the substance is to be volatilized. Troost and Devillef have de- 

 scribed a method which introduces important modifications in 

 both these points. 



They find that a constant temperature is obtained by the eva- 

 poration of substances of moderately low boiling-points, provided 

 care be taken to exclude the influence of the heat from the source 

 of heat, and that of the surrounding air. In the arrangement 

 which Deville and Troost describe, these sources of error are ex- 

 cluded. The substances used for determinations, in which glass 

 vessels can be employed, are mercury and sulphur ; the former 

 of which boils at 350° C, and the latter at 440° C. 



The apparatus consists of a mercury bottle cut off near the 

 neck, so as to form a cylinder closed at the bottom. In the in- 

 terior are two rings provided with holes, between which the glass 

 globe is held firm about G or 8 centimetres above the bottom of 



* Liebig's Annalen, December 1857. 



t Comptes Rendus, vol. xlv. p. 821. Liebig's Annalen, February 1858. 



