PROTEINS 163 



rather to salting out, which, we may assume, occurs also in the case 

 of crystalloids; it may be induced not only by electrolytes which 

 raise the melting point of the gel, but also by those which depress it. 

 Precipitation becomes evident at first through a turbidity which 

 may be sufficiently marked to give a tenacious gelatin phase and a 

 more limpid aqueous phase. In precipitations, also, the anions have 

 the determining influence, their precipitating effect being arranged in 

 the following order: 



SO 4 > Citrate > Tartrate > Acetate > Chlorid. 



Inorganic hydrosols behave quantitatively toward gelatin the 

 same as toward albumin (see p. 156). 



The swelling and shrinking of gelatin referred to on page 68, et seq., 

 are characteristic for all elastic gels. 



According to J. TRAUBE and F. KOHLER there exists a parallelism 

 between the swelling, shrinking, solidification and melting point of 

 gelatin when it is mixed with other substances. 



The tinctorial properties of elastic fibers and their chief constituent, 

 elastin, are better known than their other colloidal properties. 



To the investigations of P. G. UNNA and L. GOLODETZ, we owe our 

 knowledge of the keratins, the horny substances composing skin, 

 hair, nails, hoofs, horns, feathers, etc. Chemical studies of these 

 substances are very difficult because they resist chemical attack. 



We shall merely mention the remaining albuminoids, spongin, the 

 structural support of ordinary sponge, chonchiolin, the framework of 

 mussels and snails and further, the albumoids, a group into which 

 almost all unclassified proteins are thrown. 



Nucleoalbumins. 



These proteins, like the albumins, are digested by pepsin-hydro- 

 chloric acid; they dissolve almost entirely, but at the same time 

 split off an almost insoluble phosphorus-containing complex. The 

 casein of milk, the vitellin of egg yolk and perhaps also legumin and 

 vegetable casein are nucleoalbumins. It is remarkable that among 

 the phosphorus-containing proteins obtained from seeds, there should 

 be several that are soluble in alcohol (gliadin from cereal grains and 

 zein from corn). Because of this, it is a question however, whether 

 they are related to casein. 



On account of its' importance, casein has been most extensively 

 investigated. In milk, casein exists as a salt (united to lime and 

 alkali) and, being dissolved, exhibits profound hydrolytic dissociation. 

 Casein may be thrown out of solution by the addition of acids or ren- 



