36 PHYSICAL CHEMISTRY IN BIOLOGY. 



The urine of man and mammalia generally has a much higher 

 osmotic pressure than the corresponding blood. 1 For human urine 

 A varies between 1.3 and 2.3. After abundant drinking as well as 

 under pathological conditions (diabetes insipidus) the osmotic pres- 

 sure of the urine can be lower than the blood. In regard to the osmotic 

 pressure of animal fluids under normal and pathological conditions we 

 refer to the work of KORANYI and RicnTER. 2 



II. COLLOIDS. 



The word colloid originated with GRAHAM, who included in this name 

 different substances which did not have the property of diffusing through 

 an animal membrane. In opposition to this GRAHAM called those 

 bodies which passed through a membrane, crystalloids, because they 

 were as a rule crystalline, a property which with few exceptions does 

 not belong to the colloids. 3 GRAHAM included soluble silicic acid among 

 the colloids and also analogous forms of stannic acid, titanic acid, 

 molybdic acid and tungstic acid, the aluminium hydroxide and analo- 

 gous metallic oxides, when they exist in the soluble form, and also starch, 

 dextrins, the gums, caramel, tannin, albumin and gelatin. 



Some colloids are characterized by the fact that under certain 

 conditions they solidify in a gelatinous form containing considerable 

 water. In the case where water is the solvent then GRAHAM called the 

 soluble form hydrosol and the gelatinous form hydrogel. 



By diffusion through a membrane (called dialysis by GRAHAM) colloid sub- 

 stances can be separated from crystalloids. Colloidal silicic acid as well as 

 corresponding forms of certain other bodies are obtained by treating the soluble 

 alkali salt with hydrochloric acid, then removing the excess of hydrochloric acid 

 as well as of chlorides, by means of dialysis. Colloidal alumina was obtained by 

 GRAHAM by dissolving aluminium hydroxide in aluminium chloride. This last salt 

 was removed by dialysis and the hydroxide remained with more or less HC1 com- 

 bined in solution. 



Various metallic sulphides can be obtained in colloidal solution. Such solu- 

 tions of As 2 S 3 and Sb 2 S s can be obtained by passing H 2 S into dilute solutions 

 of the respective metallic oxide, 4 and colloidal CuS can be prepared by washing 

 the precipitated compound with water, by which treatment the CuS finally becomes 

 soluble in water. 5 



The metals can be obtained as hydrosolsj and indeed in two ways : 



1. By treating a salt with various reducing agents (for example formaldehyde, 

 hydrosulphurous acid, hydrazine, hydroxylamine) the various metals are obtained 

 in colloidal solution. 8 As the solutions thus obtained are often very unstable, 



1 Koranyi, Zeitschr. f. klin. Med., 33, 1, 1897; 34, 1, 1898. 



2 Physikalische Chemie und Medizin. Leipzig, 1907. 



3 Ann. d. Chem. u. Pharm., 121, 1, 1862, as well as Ann. de chim. et de Phys. (4), 

 3, 127, 1864. 



4 H. Schulze, Journ. prakt. Chem. (N.F.) 25, 431, 1882 and 27, 320, 1883. 



5 Spring, Ber. d. d. chem. Gesellsch., 16, 1142, 1883. 



6 Miiller, Allg. Chemie d. Kolloide. Leipzig, 1907, 6. 



