+------------------+--------------+---------------+------------------+ | Reagent. | Gelatin. | Albumins. | Keratins. | +------------------+--------------+---------------+------------------+ |Cold water |Swells only |Soluble |Insoluble. | | | | | | |Heated in water |Soluble |Coagulate at |Soluble only at | | | |72°to 75° C. |temp. over 100° C.| | | | | | |Acetic acid and }| | | | |potassium ferro- }|No precipitate|Precipitate |Precipitate | |cyanide to aque- }| | | | |ous solution }| | | | | | | | | |Millon’s reagent |No reaction |Red coloration |Red coloration. | | | | | | |Hot concentrated }|No coloration |Violet blue |No coloration. | |hydrochloric acid}| | | | +------------------+--------------+---------------+------------------+ There is no simple method for the quantitative separation of the different constituents of skin. It is, therefore, customary to simply determine the amount of nitrogen which any particular portion of the material may contain, and, as gelatinous fibre, which constitutes by far the greater portion of the true skin, contains 17·8 per cent. of nitrogen, to base the estimation of the amount of skin present upon this figure (see p. 57). The most convenient process for the determination of the nitrogen is that devised by Kjeldahl, which is most easily carried out as follows:-- A known weight of the substance which contains about 0·1 gram of nitrogen (0·5 gram of skin, or a corresponding quantity of liquor) is placed in a flask of Jena glass, capable of holding 500-700 c.c. together with 15 c.c. of concentrated sulphuric acid. The contents of the flask are then boiled over a small Bunsen flame for 15 minutes, or more, until all the water has been driven off and the material is quite disintegrated; and are then allowed to cool below 100°. 10 grams of dry powdered potassium persulphate is now added, and the boiling continued till the liquid has become colourless. The operation of boiling should be conducted in a good draught, or in the open air. Before the substance has begun to char it is advisable to place a small funnel in the neck of the flask to prevent, as far as possible, spirting and loss of sulphuric acid. [Illustration: FIG. 18.--Kjeldahl Apparatus.]