Chemical Science. 381 



and the whole success of the operation (which is pretty capri- 

 cious) depends upon it. The precipitate is to be left 24 hours 

 in the mortar, stirring it, from time to time, with a glass rod. 

 It is then to be poured into a saucer, or other similar vessel, and 

 left till the powder subsides. The supernatant liquid is then 

 drawn off with a syphon, and the deposit is dried in the shade, 

 without washing it. 



The precipitate when dried, is yellowish white. The muffle 

 in which it is to be baked, ought to be red hot. The powder is 

 put upon a silver, or porcelain plate, of the thickness of one or 

 two lines ; and it must be withdrawn from the fire the instant 

 that it acquires its purple colour. If it be left too long exposed 

 to heat, it acquires a tinge of violet. This is occasioned by the 

 salts it still contains ; for, after it has been washed, it may be 

 kept red-hot without losing any of its colour, which indeed 

 acquires greater lustre. 



These trials, it is observed, were made on a small scale, and 

 may be improved. The colour appears to want intensity, but 

 the mixture of oils, or gum, renders it sufficiently dark. For 

 oil-painting, it must be carefully rubbed with a mixture of 

 drying oil and varnish. The painting is to be begun by a thin 

 transparent coat. A second coat is sufficient to give it all the 

 lustre of which it is susceptible. The under coats ought to be 

 prepared with rough terra de sienna. — Annals of Philos. xiv. 

 p. 361. 



16. Economical Mode of Rectifying Spirit of Wine. — It is well 

 known that water passes with facility through animal mem- 

 branes, as bladder, whilst alcohol is almost perfectly re- 

 tained by them. Thus, by closing a bottle of wine by a piece 

 of bladder, instead of a cork, the wine is, after some time, found 

 diminished in quantity, but strengthened in quality. The Giornale 

 di Fisica proposes this principle to be adopted in the rectifi- 

 cation of spirits, and relates an experiment made by a corre- 

 spondent. If alcohol of 30° (S.G. ,867) be put into a bladder 

 until it is half full, the orifice closed, and the bladder then 

 exposed to the sun, the air, or the heat of a stove, after a short 



Vol. VIM. 2C 



