1884.] NATURAL SCIENCES OF PHILADELPHIA. 27 



that unless kept in dust-proof cases they would become spoiled 

 by the dust collecting on them. 



As absolute alcohol is so expensive in this country, the cost of 

 a large specimen would be considerable, and therefore the process 

 is better adapted for smaller objects. 



A cheap method of making absolute alcohol, from the strong 

 (95 per cent.) spirit, used in Prof. L. Rauvier's laboratory in 

 Paris, would not, I think, be out of place to be mentioned here. 



The details of this process were given me by my friend, Dr. 

 W. Yignal, the assistant of Prof. Rauvier. A wide-mouthed 

 bottle is takeji, holding about a litre, and a three-quarters filled 

 with the strong alcohol. 



A mass of pulverized cupric sulphate (CuSO^ + 5 Aq.) is 

 heated to a red heat in order to drive off the water of crystalliza- 

 tion. This is poured, when cool, into the alcohol, the mouth of 

 the bottle quicklj^ closed, and the whole shaken. The cupric 

 sulphate is insoluble in alcohol, but has an affinity for the water 

 contained in it, and the water is consequently taken up, and the 

 cupric sulphate becomes bluish. When this has stood — with 

 occasional shakings — for a day or so, decant, and repeat the 

 operation, especially if there is very much of a blue color in the 

 sediment. 



When finished a drop of alcohol can be mixed with a drop of 

 turpentine on an object-glass, and if there be no particles of water 

 to be seen under the microscope, the alcohol is absolute enough 

 for all practical purposes. 



