General Science. 
411 
in texture, density and polish, is said to be practised in France 
with such success, that the best judges are incapable of distin- 
guishing between the imitation and the mahogany. The sur- 
face is first planed smooth, and the wood is then rubbed with a 
solution of nitrous acid. One ounce and a-half of dragon’s 
blood, dissolved in a pint of spirits of wine, and one-third of an 
ounce of carbonate of soda, are then to be mixed together and 
filtered, and the liquid in this thin state is to be laid on with a 
soft brush. This process is repeated, and in a short interval af- 
terwards the wood possesses the external appearance we have de- 
scribed. When the polish diminishes in brilliancy, it may be re- 
stored by the use of a little cold-drawn lintseed oil .— London 
Journal of Arts, vol. iv. p. 107. 
56. Etching on Glass by Fluoric Acid. — Professor Silliman, in 
a paper on this subject, strongly recommends the diluted fluoric 
acid of Gay Lussac, in preference to the vapour, as being en- 
tirely superior in energy, neatness and ease of management. 
The strong acid is violent and dangerous in the extreme, and 
should never be allowed to touch the skin, either in the fluid or 
vaporous state ; but the diluted acid may be managed with ease 
and safety. Still, however, even this sometimes produces sores 
that last for six weeks. The following is the method of obtain- 
ing the acid. Two ounces of pure fluor-spar were introduced into 
the alembic, and four ounces of sulphuric-acid were added. The 
apparatus was placed under a flue. The receiver (which was of 
silver) was kept cold by ice, and when a few live coals was 
placed beneath the alembic, the acid was disengaged, and was 
condensed in the receiver without the aid of water. About an 
ounce of acid was thus obtained. When the acid was wanted 
for the purposes of etching on glass, the same method was used, 
only an ounce of water was placed in the silver receiver. This 
acid was still too powerful, as it corroded the varnish used to 
protect the glass, but it acted in the happiest manner when di- 
luted with three or four parts of water. The best varnish for 
this purpose is made by melting together common turpentine 
and bees’ wax. — Amer . Journ. of Science , vol. vi. No. ii. p. 654. 
57. Important Improvement in Tanning.- — Mr Gybbon Spils- 
bury of Walsale, Staffordshire, we .understand, has succeeded in 
