740
LACTOMETER.
He next filtered, evaporated to dryness on a water bath, and digested the residuum **strong alcohol, which dissolved the lactic acid, and left the sugar of milk. On evapora-ting off the alcohol, the acid was obtained. As thus procured, it requires to be purifiedby saturation with carbonate of lead (pure whjte lead), and precipitating the solution oithis lactate with sulphate of zinc, not added in excess. Sulphate of lead falls, and thesupernatant lactate of zinc being evaporated affords crystals, at first brown, but whichbecome colorless on being dissolved and re-crystallized twice or thrice. If the sulphur* 1 -acid of the dissolved salt be thrown down by water of baryta, the liquid when filtered andevaporated yields a pure lactic acid, of a sirupy consistence, colorless and void of smeu-It has a pungent acid taste, which it loses almost entirely when moderately diluted withwater. It does not crystallize. Its salts, with the exception of those of magnesia anzinc, have a gummy appearance, and are very soluble in alcohol, unless they hold an ex-cess of base. Lactic acid consists of 44-92 carbon; 6-55 hydrogen ; 48-53 oxygen. *contains 9-92 per cent, of water. It has not hitherto been applied to any use in the arts,except by the Dutch in their old process of bleaching linen with sour milk. ,
LACTOMETER is the name of an instrument for estimating the quality of m*fi'>called also a Galadomcler, which see. The most convenient form of apparatus would b<ja series of glass tubes each about 1 inch in diameter, and 12 inches long, graduatethrough a space of 10 inches, to tenths of an inch, having a stop-cock at the bottom, a ®suspended upright in a frame. The average milk of the cow being poured in to 't*height of 10 inches, as soon as the cream has all separated at top, the thickness ot *-body may be measured by the scale; and then the skim milk may be run off het°into a hydrometer glass, in order to determine its density, or relative richness in case**matter. . e
LAKES. Under this title are comprised all those colors which consist of a vegetadye, combined by precipitation with a white earthy basis, which is usually alumina. 4general method of preparation is to add to the colored infusion a solution of co»nBnalum, or rather a solution of alum saturated with potash, especially when the *" .has been made with the aid of acids. At first only a slight precipitate falls, consist* sof alumina and the coloring matter; but on adding potash, a copious precipitation ens I^ e jof the alumina associated with the dye. When the dyes are not injured, but are rat ^brightened by alkalis, the above process is reversed; a decoction of the dye-stuff * swith an alkaline liquor, and when it is filtered, a solution of alum is poured into it. (ethird method is practicable only with substances having a great affinity for subsulp ^of alumina; it consists in agitating recently precipitated alumina with the decoctionthe dye. e
Yellow lakes are made with a decoction of Persian or French berries, to which so ^potash or soda is added; into the mixture a solution of alum is to be poured as lon | eS ,any precipitate falls. The precipitate must be filtered, washed, and formed into caand dried. A lake may be made in the same way with quercitron, taking the preenu gto purify the decoction of the dye-stuff with buttermilk or glue. After filtering th^ |heit may be brightened with a solution of tin. Annotto lake is formed by dissolv*®-, ( . ndye-stuff' in a weak alkaline ley, and adding alum water to the solution. Solutiongives this lake a lemon yellow cast; acids a reddish tint.
Red lakes.- —The finest of these is carmine. ffe
This beautiful pigment was accidentally discovered by a Franciscan monk at ” isa jj c ; n e,.formed an extract of cochineal with salt of tartar, in order to employ it as a D1 V,j 6 j ie dand obtained, on the addition cf an acid to it, a fine red precipitate. Homberg_pu _a process for preparing it, in 1656. Carmine is the coloring matter of /J 3 aMordingpared by precipitation from a decoction of the drug. Its composition varies acc 0 fthe mode of making it. The ordinary carmine is prepared with alum, and con g p6earminium (see Cochineal), a little animal matter, alumina, and sulphuric aciCabmine. . j,rjnent>
Carminated lake, called lake of Florence, Paris , or Vienna . For mtking *h*® P = t jds,the liquor is usually employed which is decanted from the carmine process. not toonewly precipitated alumina is put; the mixture is stirred, and heated a little, se ttlo>much. Whenever the alumina has absorbed the color, the mixture is alloweand the liquor is drawn off'. . , gn a dded,
Sometimes alum is dissolved in the decoction of cochineal, and potash , . ^. a y anto throw down the alumina in combination with the coloring matter; but in , :„pten th eindifleient pigment is obtained. Occasionally, solution of tin is added, to•lye. . ,. but now it
A lake may be obtained from kermes, in the same way as from cochinea >is seldom had recourse to. • for l'°
Brazil-wood lakes.— Brazil wood is to be boiled in a proper quantity and »
minutes ; then, alum and solution of tin being added, the liquor is to , b . para tea hi'solution of potash poured in as long as it occasions a precipitate. This is