334 CHEMISTRY OF THE PROTEIDS CHAP. 



recrystallised three to six times to remove all impurities. If albuminous 

 substances are denaturalised by means of alkali, then all previously 

 existing differences disappear ; all alkali-albuminates have the same low 

 gold number. 



6. Power of forming Emulsions 



Owing to their colloidal l nature, albumins possess the power of 

 keeping insoluble substances in solution. In this way lecithin and 

 calcium-soaps are kept in solution in serum and calcium-phosphate in 

 milk. According to Paal, 2 albuminates have further the power of 

 dissolving metallic oxides. In this connection may also be mentioned 

 the older observations of Schadee van der Does, 3 who described how 

 solutions of egg- and serum-albumin may be rendered uncoagulable 

 by being shaken up with freshly prepared metallic silver in fine sub- 

 division, or freshly prepared or not too old silver oxide. The author 4 

 explains this fact by assuming the silver oxide to act similarly to 

 osmium tetroxide, after the addition of which egg-albumin does not 

 coagulate on heating (Bethe and Monckeberg). 5 In the case of 

 osmium, the Os0 4 is probably changed into Os(OH) 4 , which replaces 

 the displaceable hydrogen-atoms of the albumin-molecule. The author 

 arrived at his interpretation by assuming that during heat-coagulation 

 a potential hydrogen-ion is liberated, and that this H-atom in the 

 albumin-molecule is oxidised by Os0 4 . That such a replaceable 

 H-atom really does exist has been shown since by the researches of 

 Heffter 6 (see p. 97). The author's explanation is thus a chemical one, 

 and shows that the solution of metallic oxides does not depend on the 

 ' colloidal ' nature of albumins as defined by Cohnheim. 



It is quite different, however, with mixtures of two or more 

 colloids ; for one colloid being impermeable to and by another colloid 

 it is easy to understand why albumins and albumoses remain in 

 solution in albuminous fluids, although they are insoluble in non- 

 albuminous fluids, for each colloid interferes mechanically with the 

 precipitation of the other colloid (Mann). This very property makes 

 it so difficult to isolate the albumins. We must, however, always keep 

 in mind the fact that both salts and other radicals of the chemi- 

 cally active albumins play a part in this process. Related to these 

 phenomena is, further, the power possessed by casein and other 



1 ' Colloidal ' includes definite chemical action, as explained on pp. 259 and 268. 

 ' 2 C. Paal, Ber. d. deutsch. diem. Ges. 35. II. 2206 (1902). 



3 S. v. d. Does, Zeitschr.f. Physiol. Chem. 24. 351 (1897). 



4 Mann, Physiological Histology, 1902, p. 67. 



5 Bethe and Monckeberg, Arch. f. mikr. Anat. 54. 135 (1899). 



6 A. Heffter and Max Hausmann, Hofmeisters Beitrage, 5. 213 (1904). 



