854 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



residue is tlien to be treated with alcohol, having the strength of 

 about cighty-thiee per cent. This process can be performed by 

 an}^ farmer's daughter, but when the amount to be made is very 

 larire, the oi^ of almonds should be added to the saturated bisul- 

 phide, and the whole distilled at a very low temperature, so as to 

 save the bisulphide, and the residue is treated with alcohol, as before 

 described. 



Dr. Hirsh remarked in relation to this item that the odor of . 

 bisulphide of carbon was objectionable, and it was found more 

 convenient to use benzole. 



Fire Damp. 



The cause of explosions in coal mines so destructive in Great 

 Britain, is the accidental ignition of air containing light carburetted 

 hydrogen, or garol, which the miners call fire damp — a gase- 

 ous compound, consisting of twelve parts by weight of carbon, and. 

 four parts of hydrogen. In the process of mining, crevices are 

 opened containing this gas, which, by the law of diftusion, soon 

 becomes equally mixed with the whole air of the mine. It is well 

 knoAvn that diaphragms of animal matter possess the property of 

 absorbino- this gas hy endosmosis. An empty bladder, for 

 instance, suspended in a mixture of air and marsh gas, will soon 

 be found partially filled with marsh gas. Unglazed earthernware 

 may be substituted for the animal membrane. Mr. G. F. Ansell, 

 of the Royal Mint, London, first applied the principle of osmosis 

 in a very ingenious instrument for the detection and quantitative 

 estimate of explosive gases, which can be used in mines to give 

 warning of the presence of marsh gas in such quantities as to form 

 an explosive mixture. For certain reasons, which need not here 

 be explained, the difiiision of gases is not always equal. Several 

 of these instruments, at different heights in the gapie mine, show 

 that tliere may be a mucli larger percentage of marsh gas at the 

 top than at the bottom of the gallery. Mr. Ansell has lately 

 improved the apparatus constructed by him in 1865. It now con- 

 sists of an iron funnel, provided with an iron U-tube, the end of 

 which is closed by a piece of glass tube fixed in brass, to which 

 one pole of a battery is attached ; the upper part of this glass tube 

 carries a brass collar, through which passes an adjusting screw, to 

 the lower end of which is fastened a piece of copper wire with a 

 platinum point. Mercury is poured into the iron funnel till it 

 rises in the glass tube to a convenient height. This mercury is 



