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CLAY.

If any citrate of lime be left undecomposed by the sulphuric acid, it will dissolve in thecitric acid, and obstruct its crystallization, and hence it will be safer to use the slightestexcess of sulphuric acid, than to leave any citrate undecomposed. There should not how-ever be any great excess of sulphuric acid. If there be, it is easily detected by nitrateof barytes, but not by the acetate of lead as prescribed by some chemical authors ; be-cause the citrate of lead is not very soluble in the nitric acid, and might thus be confound-ed with the sulphate, whereas citrate of barytes is perfectly soluble in that test acid.Sometimes a little nitric acid is added with advantage to the solution of the colored crys-tals, with the effect of whitening them.

Twenty gallons of good lemon juice will afford fully ten pounds of white, crystals ofcitric acid.

Attempts were made, both in the West Indies and Sicily , to convert the lime and lemonjuice into citrate of lime, but they seem to have failed through the difficulty of drying thecitrate for shipment.

The crystals of citric acid are oblique prisms with four faces, terminated by dihedralsummits, inclined at acute angles. Their specific gravity is 1-617. They are unalter-able in the air. When heated, they melt in their water of crystallization; and at ahigher heat, they are decomposed. They contain 18 per cent, of water, of which onehalf may be separated in a dry atmosphere, at about 100° F., when the crystals fall intoa white powder.

Citric acid in crystals is composed by my analysis of carbon 35-8, oxygen 59-7, andnydrogen 45; results which differ very little from those of Dr. Prout, subsequently ob-.ained. I found its atomic weight to be 8-375, compared to oxygen 1,000. I cannot ac-count for Berzelius ’s statements relative to the composition of this acid.

Citric acid in somewhat crude crystals is employed with much advantage in calico-printing. If adulterated with tartaric acid, the fraud may be detected by adding potashto the solution of the acid, which will occasion a precipitate of cream of tartar.

CIVET. ( Civette , Fr. ; Zibeih, Germ.) This substance approaches in smell tomusk and ambergris; it has a pale yellow color, a somewhat acrid taste, a consistencelike that of honey, 'and a very strong aromatic odor. It is the product of two smallquadrupeds of the genus viverra (v. zibetha and v. civetta), of which the one inhabitsAfrica , the other Asia. They are reared with tenderness, especially in Abyssinia.The civet is contained in a sac, situated between the anus and the parts of generationin either sex. The animal frees itself from an excess of this secretion by a contractilemovement which it exercises upon the sac, when the civet issues in a vermicular form;and is carefully collected. The negroes are accustomed to increase the secretion by irri-tating the animal; and likewise introduce a little butter, or other grease, by the naturalslit in the bag, which mixes with the odoriferous substance, and increases its weight. I*is employed only in perfumery.

According to M. Boutron-Chalard, it contains a volatile oil, to which it owessmell, some free ammonia, resin, fat, an extractiform matter, and mucus. It affords, Wcalcination, an ash, in which there are some carbonate and sulphate of potash, phosphat eof lime, and oxyde of iron. ,

CLAY (Jlrgile, Fr.; Than, Germ.) is a mixture of the two simple earths, alumina an“silica, generally tinged with iron. Lime , magnesia , with some other coloring metalh coxydes, are occasionally present in small quantities in certain natural clays.

The different varieties of clay possess the following common characters :— _

1. They are readily diffusible through water, and are capable of forming with it a pi® '

tic ductile mass, which may be kneaded by hand into any shape. This plasticity existshowever, in very different degrees in the different clays. . g

2. They concrete into a hard mass upon being dried, and assume, upon exposure to t

heat of ignition, a degree of hardness sometimes so great as to give sparks by colli 51with hardened steel. In this state they are no longer plastic with water, even when Pverized. Tolerably pure clays, though infusible in the furnace, become readily so byadmixture of lime, iron, manganese, &c. j-

3. All clays, even when previously freed from moisture, shrink in the fire in v ' rtue ( j r vthe reciprocal affinity of their particles; they are very absorbent of water in theirstate, and adhere strongly to the tongue.

4. Ochrey, impure clays, emit a disagreeable earthy smell when breathed upon.

Brongniart distributes the clays into :—

1. Fire-clays, ( argiles apyres, Fr.; feuerfeste , Germ.)

2. Fusible, ( schmelzbare , Germ.)

3. Effervescing ( brausende , Germ.), from the presence of chalk.

4. Ochrey (ocreuses ,, Fr.; ockrige, Germ.) . , r pur-

Fire-clay is found in the greatest abundance and perfection for manufacturing

poses in, ivrassi'' , >

1. Slate-clay. (Thwi-schiefer, Germ.) Its color is gray or grayish-yellow. 1