TRANSACTIONS OP THE SECTIONS. 49 



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 con- 

 taining magnesia, in cases where, from the slowness of their decomposition, they con- 

 tinue sterile ; and proposed in such cases to accelerate their disintegration, by pouring 

 upon the subsoil diluted sulphuric acid. 



Practical Method of determining the Quantity of Real Indigo in the Indigos 

 of Commerce. By Dr. Samuel L. Da^ a, of Lowell, 3Iassachusetts, U.S. 



Dr. Dana is chemist in the most extensive calico-printing works of the United 

 States. Not less than 50,000 lbs. of indigo are consumed annually in that establish- 

 ment. Hence the importance of guiding the purchasers of that article by the results 

 of easily performed analyses of samples. The following is the method employed by 

 Dr. Dana, and which he requested might be laid before the Chemical Section of the 

 British Association : — 



1. Reduce the indigo to an impalpable powder. 



2. Boil, in a long-neck flask (a Florence flask will do), ten grains of indigo, a few 

 minutes, in 2| oz. of a solution of carbonate of soda, marking 30° to 35° on Twaddel's 

 hydrometer ; then add eight grains, in crystals, of muriate of tin, and boil for half an 

 hour. A beautiful yellow solution of indigo will be obtained. Withdraw from the 

 lamp, and, 



3. Introduce into the solution (2.) 500 water-grain measures of a solution of fifty 

 grains 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. 



4. Filter the precipitate (3.) through a double weighed filter, washing the mass with 

 one ounce of muriatic acid diluted with three ounces of boiling water ; wash with 

 hot water till only water returns. 



5. Separate, dry and weigh the filters ; note the weight of the precipitate; burn one 

 filter against the other ; weigh ; the difference is the silica contained in the indigo, 

 which, deducted from the weight of the precipitate, gives the quantity of pure indigo. 



Dr. Dana states that he has verified the accuracy of this mode by repeated results 

 obtained in the dye-house ; the indigo spending exactly in proportion to the quantity 

 of that substance as found by analysis. He has also been able to use much more 

 productive indigos since the agent of his house in Calcutta has been instructed by the 

 results of these experiments. 



Mr. Walter Crum, who communicated the above, adds the remark : — Carbonate of 

 soda with protoxide of tin does dissolve indigo and forms a yellow solution, but so 

 slowly, that I doubt if all the ten grains are acted upon. I think 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. 



Some Experiments shoioing the possibility of Fire, from the use of Hot Water 

 in warming Buildings, and of explosions in Steam-engine Boilers. By 



GOLDSWORTHY GuRNEY. 



After detailing several instances of fire which arose from the steam pipes of water 

 apparatus used for warming houses, the author proceeds to describe some of the ex- 

 periments likely to be of practical value. From a tubular boiler, driving a high-pres- 

 sure engine, the injection pump was cut off; half an hour after the supply pump was 

 stopped, no water appeared on opening the gauge cocks, and the engine was observed 

 to slacken its rate and to move sluggishly ; it had dropped from 50 to 30 strokes a 

 minute. The steam pipe from the boiler to the engine was 40 feet long, and was 

 carried for convenience through the open air, thickly wrapped round with woollen 

 cloth to prevent radiation : soon after the engine became sluggish, the woollen cloth 

 was observed to char near the boiler, an effect which soon extended along the whole 

 length of pipe ; the engine still working, but with more apparent difficult}-, making 

 only 16 strokes per minute ; the pressure gauge, which usually ranged between 30 and 

 40 pounds, now stood at 15, and was gradually sinking. In about five minutes after 

 the woollen cloth had charred, a lead flange, used as a packing at the cylinder joint, 

 1841. E 



