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is foluble in water, but much lefs eafily than thetannin, and hence if the catechu in fine powderbe repeatedly and for a fhort time wafhed withwater, all the tannin is difiblved (which isknown by no longer precipitating the iolution ofglue) and alfo moft of the mucilage, and therefidue is a pale red extraft, only flightly aftrin-gent, but confiderably fweet when chewed. Itdiflolves both in water and alcohol, acquiring adeeper colour by expofure to air.
Catechu therefore confifts chiefly of—ift. avery large portion of tannin, to which it owesits aftringency, and the very great powers whichit poflefl'es as a material for tanning leather:2d. of a peculiar extract, in which refides thefweetnefs of tafte for which this extract is valuedin the Eaft: and 3d. a fmall portion of mucilage,inert and infipid. Mr. Davy found that 200grains of the Bombay catechu contained 109grains of tannin, 68 of extract, and 13 of mu-cilage, with xo of earthy refidue. The famequantity of the Bengal catechu gave of the fameingredients 97 grains, 73, 16, and 14.
Catechu is ufed in the Eaft, as it is faid, formany procefles of dyeing, and would doubtlefsanfwer in many cafes where galls are now ufed.But its chief confumption is as an ingredient inthe betel compofition, which is fo univerfallychewed. It is ufed in this country only inmedicine, as a very valuable aftringent, but itstanning powers deferve farther trial.
CAUSTICITY. Aetjbarleit , Germ.
A fubftance is ufually faid to be cauftic whenit produces the fame effeft to the tongue as thatof aftual fire, that is, an immediate fenfation ofburning, followed with a flight diforganizationof the furface actually in contact. Thus thealkalies are called caujlic when deprived of car-bonic acid, becaufe, when at all concentrated,they then burn and blifter the tongue almoltinftantly.
Cauftic fubftances are alfo generally corrofive,or fuch as aft upon organized matter and de-compofe it with great rapidity.
The term cauftic prefixed to the alkalies andearths to diftinguifh the pure or de-carbonatedftate, is omitted in the modern nomenclature,being rendered unneccffary by the ufe of theterm carbonat; thus to the terms caujlic potajfjand mild potajh are fubftituted thofe of potajhand carbonat of potnfj refpeftively. Some con-fufion {fill arifes cccafionaily from this change,the term foda for example, meaning in chemicallanguage pare or caujlic foda, and in commerceand common ufe meaning the mild, carbonat of
VOL. 1.
foda. In this work where the carbonat is meant,it is always fo exprefled.
CAUSTIC Lunar is the nitrat of Silvermelted and eaft into cylindrical pieces about thefize of common flate-pencils, for the ufe of fur-geons.
CEMENTATION. Camentiren , Germ.
This term is applied to a procefs in the dryway fimilar to digeftion in the moift, and meansthe expofure of any fubftance to a regular fur-nace-heat in a crucible, ftratified, or otherwifecovered, with fome kind of powder, which isintended to produce a chemical change. Thusiron bars are converted into fteel by being ce-mented with a powder of bone-afh and othermaterials : copper into brafs by cementation witha powder of calamine and charcoal, and the like.The powder is in this caie called Cement-powder.
CEMENT-COPPER. The copper procuredfrom the fulphat by precipitation with iron is focalled. See Copper.
CEMENTS and LUTES. Under this articlemay be mentioned the receipts- for preparingfome of the moft ufeful fubftances of this kindthat are required in common chemical opera-tions.
The ufes of lutes and cements are, either toclofe the joinings of chemical veflels to preventthe efcape of vapours and gaffes during the pre-cedes of diftillation, fublimation and the like:or to proteft veflels from the aftion of the firewhich might crack, or fufe, or calcine them: orfometimes to repair flaws and cracks, and for avariety of other fmaller purpofes.
The fubjeft of calcareous cements, fuch asmortar, tarras, and other fubftances ufed to clofethe joinings of bricks or ftones in buildings,will be mentioned in the following article.
When a lute is applied over the whole fur-face of a veflel (as to a glafs retort when it isintended to be heated red-hot) the procefs istermed lorication or coating. Iron furnaces arealfo lined or coated on the infide with earth toprevent the iron from being deftroyed by theconftant aftion of the fire.
Prom the vaft variety of receipts for lutes andcements of different kinds the following may befelefted, which will anfwer moft of the pur-pofes of the experimental chemift.
To prevent the elcape of the vapors of water,fpirit, and liquors not corrofive, the fimple ap-plication of flips of moiftened bladder willanfwer very well for glafs, and paper with goodpafte for metal. Bladder to be very adhefivefliould be foaked fome time in water, moderatelywarm, till it feels clammy, it then fticks very2 M 1 '