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Proceedings of the British Association. 321 
yellow solution of indigo will be obtained. Withdraw the flask 
from the lamp, and introduce into the solution 500 water-grain 
measures of a solution of 50 grains of bichromate of potash in 
4000 grains of water. The indigo blue, with a trace of indigo 
red, will be precipitated, while the other components remain in 
solution. Filter the precipitate through a double weighed filter, 
washing the mass with 1 oz. of muriatie acid, diluted with 3 oz. 
of boiling water; wash with hot water till water only returns; 
separate, dry and weigh the filters; note the weight of the pre- 
cipitate, burn one filter against the other; the difference is the 
silica contained in the indigo, which deducted from the weight 
of the precipitate, gives the quantity of pure indige. Mr. Walter 
Crum, who communicates the above, adds, that carbonate of soda 
With protoxide of tin does dissolve indigo, and forms a yellow 
solution, but so slowly that he doubts if all the ten grains are 
acted upon. He thinks Dr. Dana must mean soda-ash, which 
contains a notable quantity of caustic soda; but a much weaker 
solution of caustic soda would answer the purpose. 
On the disintegration ef the Dolomitie Rocks of the Tyrol, by 
Prof. Daubeny. The author attempted to explain, without re- 
sorting to volcanic agency, the abrupt form, extraordinary height, 
naked outline, and fissured surface of the dolomitic rocks of the 
Tyrol. He attributed these characteristics to the slow rate at 
which decomposition proceeds in rocks consisting of pure dolo- 
mite, and the'strength of the cohesion which binds together the 
particles of this rock, owing to which, even those portions which 
stand prominent in consequence of the removal, by the agents of 
destruction, of their contiguous parts, often remain unaffected by 
those mechanical forces which would cause the projecting por- 
tions of a rock Jess unyielding in its texture, to become detached. 
The cause therefore of the greater height maintained by the do- 
lomites of the Tyrol, than by the pyroxenic rocks which accom- 
pany them, seems to be the inferior rate at which decomposition 
has proceeded in the former, whilst the bold and jagged outline 
they display, may have been produced by the tenacity with which 
their parts cohere. The sterile character of these same rocks, 
even in parts which are not precipitous, appears to be owing to 
the slowness with which they decompose, as well as perhaps to 
the absence of organic remains. The Professor concluded with 
some suggestions as to the means of fertilizing rocks containing 
1 
Vol. x11, No. 2.—Jan.-March, 1842. 
