through a capillary tube.[30] The firmness of a jelly, which is often important for commercial purposes, is frequently measured by Lipowitz’s method, in which a slightly convex disc, conveniently of exactly 1 cm. diameter, and cemented to the bottom of a thistle-head funnel tube, is loaded gradually with mercury till it sinks in the jelly. The jelly (5 or 10 per cent.) should be allowed to set some hours before the test is made. [29] See Pascheles, ‘Versuche über Quellung,’ Archiv für ges. Path., Bd. 71. [30] See Prollius, Ding. Polyt. Journ., ccxlix. p. 425, who employs a 1 per cent. solution; also Stützer, Zeit. Ann. Ch., xxxi. pp. 501-15. Solutions of gelatin from skin and bone are powerfully lævorotatory to polarised light. At 30° C. (A)_{D} = -130°, but temperature and the reaction of the solution have much influence on the value found. Gelatin is precipitated from aqueous solution by the addition of strong alcohol and concentrated solutions of ammonium sulphate and some other salts. Many other colloid bodies such as dextrin and gums behave similarly. In the absence of these substances, precipitation by alcohol may be utilised for the technical analysis of gelatins and glues, printers’ roller compositions and gelatin confectionery. 25 c.c. of the gelatinous solution, which is preferably of about 10 per cent., is placed in a small beaker tared together with a glass stirring rod, and thrice its volume of absolute alcohol added. On stirring, the gelatin sets firmly on the rod and sides of the beaker, and may be washed with dilute alcohol or even with cold water, dried and weighed. A very pure French gelatin gave 98·6 per cent., while a common bone-glue only yielded about 60 per cent. precipitate. Absolute alcohol withdraws water from gelatin-jelly, leaving a horny mass. Gelatin may also be precipitated completely by saturating its solution with sodium chloride, and then acidifying slightly with sulphuric or hydrochloric acid; and masses of jelly become hardened in acidified salt solution as in alcohol, though a neutral solution has little effect. The cause of this is difficult of explanation, but its bearing on the pickling of sheep-skins (p. 89) and the production of white leather (p. 186) is obvious.