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Behavior of Gelatin — Sheppard
709
COOH), and can therefore combine with acids through the amino group and combine with alkaHes through the carboxyl group. There has been much debate among chemists as to whether gelatin really forms definite chemical compounds with acids and bases or whether it forms only indefinite so-called absorption compounds. It is not necessary for us to take sides in this matter, but we may at once note that the theory of definite chemical combination has yielded
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Fig. 1. Tlie Swelling of Gelatin in Acid and Alkali.
useful quantitative expressions for determining the degree of swelKng of gelatin in solutions of various acids and bases. The explanation of the swelling of gelatin in acid solution on this view is as follows :
It is assumed that the gelatin combines with the acid, say hydrochloric, forming a salt-like body which we may call gelatin chloride. This body is supposed to ionize into a large poorly diffusing gelatin ion and a readily diffusing chloride ion. The firm gelatin is supposed to have in addition a framework, something like a sponge, of nonionized iso-electric gelatin, and at any given concentration of the hydrochloric acid there will tend to be a definite amount of the neutral gelatin colloid and of the products of dissociation of this, the positive gelatin ion, and the negative chloride ion. It is further assumed that the diffusible chloride ions imprisoned in the gelatin framework effect an osmotic pressure and that the gelatin will con