420 Biographical Account of (Jone, 
those which usually contain that combustible. (Jour. de Min. 
xxxii. 361.) 
Descotils, whose views were chiefly directed towards metallurgy, 
had been struck by the enormous loss sustained when galena is de- 
composed by the common way. He examined (in the Memoirs 
d’Arcueil, ii. 424) the influence of the gases on the decomposition 
and volatilization of this mineral. He has given the theory of the 
operation ; and has shown that advantage would result from decom- 
posing galena by a substance which would absorb the sulphur with- 
out producing any gaseous body. 
Water mortar had been the object of the researches of several 
chemists. Bergman ascribed the property which it had of drying 
under water to the presence of about two per cent. of oxide of 
manganese. Guyton had observed that different substances gave 
lime the property of drying under water: he had even pointed out 
a method of making water mortar artificially, by calcining together 
a mixture of four parts clay, six parts of black oxide of manganese, 
and 90 parts of good lime-stone, reduced to powder. (Ann. de 
Chim. xxxvii. 259.) But Descotils ascribes the property chiefly to 
the silica, which occurs in considerable proportion in the lime-stones 
that form water mortar. He made the important remark that the 
silica does not dissolve in acids before the calcination of the lime- 
stone, while after that calcination it dissolves almost entirely. These 
results, while they give an exact idea of water mortar, point out 
the way of forming it artificially, and explain the solidity which it 
acquires so speedily under water. (Jour. de Min. xxxiv. 308.) 
Descotils had employed himself much in the examination of 
alum, and the different sulphates of alumina. He had obtained 
results which he informed us were very remarkable; but he has 
left nothing in writing upon the subject. It was while engaged 
with alum that he passed a current of chlorine through sulphate of 
alumina and ammonia, and discovered the chloride of azote, the 
existence and nature of which was first announced by M. Dulong. 
He had observed the property which this fulminating compound 
has of becoming solid by application of cold, but had deferred a 
particular investigation of it. (Jour. de Min. xxxiii. 351.) 
As Chief Engineer of Mines, Descotils went on several impor- 
tant missions. He visited the famous alum mines of Tolfa, on 
which he made a set of observations, which will appear in the first 
volume of the Annales des Mines. We regret much that he was 
unable to execute a project which he had conceived and begun to 
realise ; we mean the publishing of a treatise on docimastic chemis- 
try. He would have inserted in it a multitude of isolated remarks 
and facts, Besides, such a work is entirely wanting, and nobody 
was better qualified to execute it. 
To much general knowledge, Descotils joined a very solid judg- 
ment. If he did not perform so much as he promised, the reason 
was that he was threatened for more than 10 years with the crue) 
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