Proceedmgs of the Btitish 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 muriatic 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 indigo. Mr. Walter 

 Crura; 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 of the Dolomitic 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 less 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 



Vol. XLii, No. 2.— Jan. -March, 1842. 41 



