414 SCIENTIFIC RECREATION^. 



lead in vinegar. A solutin of this salt makes the goulard water so familiar 

 to all. Acetate of lead is highly poisonous. 



Acetate of copper is verdigris, and poisonous. Other acetates are used 

 in medicine. 



We may pass quickly over some other acids. They are as follows : 



TARTARIC ACID (C 4 H 6 O 6 ) is contained in grape juice, and crystallizes 

 in tabular form. The purified powdered salt is cream of Tartar. 



CITRIC ACID (C 6 H 8 O 7 ) is found native in citrons and lemons, as well 

 as in currants and other fruits. It is an excellent anti-scorbutic. 



MALIC ACID (G 4 H 6 O 5 ) is found chiefly in apples, as its name denotes 

 (malum, an apple). It is prepared from mountain-ash berries. 



OXALIC ACID (C 2 H 2 O 4 ). If we heat sugar with nitric acid we shall 

 procure this acid. It is found in sorrel plants. 



TANNIC ACID (C 27 N 22 O I7 ). It is assumed that all vegetables with an 

 astringent taste contain this acid. Tannin is known for its astringent 

 qualities. The name given to this acid is derived from the fact that it 

 possesses a property of forming an insoluble compound with water, known 

 as leather. Tanning is the term employed. Tannin is found in many 

 vegetable substances, but oak bark is usually employed, being the cheapest. 

 The " pelts," hides, or skins, have first to be freed from all fat or hair by 

 scraping, and afterwards soaking them in lime and water. Then they are 

 placed in the tan-pit between layers of the bark, water is pumped in, and 

 the hides remain for weeks, occasionally being moved from pit to pit, or 

 relaid, so as to give all an equal proportion of pressure, etc. The longer 

 the leather is tanned it may be a year the better it wears. 



Skins for gloves and binding are tanned with " sumach," or alum and 

 salt. Sometimes the leather is split by machinery for fine working. 

 Parchment is prepared from the skins of asses, sheep, goats, and calves, 

 which are cleaned, and rubbed smooth with pumice stone. 



Tannic acid, with oxide of iron, produces Ink, for the gall-nut contains 

 a quantity of the acid. All the black inks in use generally are composed of 

 green vitriol (sulphate of iron) in union with some astringent vegetable 

 matter ; the best is the gall-nut, although, for cheapness, logwood and oak 

 bark have each been used. An excellent black ink may be made by 

 putting into a gallon stone bottle twelve ounces of bruised galls, six ounces 

 of green vitriol, and six of common gum, and filling up the bottle with rain 

 water ; this should be kept three or four weeks before using, shaking the 

 bottle from time to time. 



Blue ink. has lately been much used ; it is made by dissolving newly- 

 formed Prussian blue in a solution of oxalic acid. To make it, dissolve 



