14 GENERAL AND PHYSICO-CHEMICAL. 



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

 molybdic acid and tungstic acid, aluminium hydroxide and analogous 

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

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



Some colloids are characterized by the fact that under certain con- 

 ditions they solidify into 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 AsaSs and Sb 2 S 3 can be obtained by passing H 2 S into dilute solutions 

 of the respective metallic oxide, 1 and colloidal CuS can be prepared by washing 

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

 soluble in water. 2 



The metals can be obtained as hydrosols, and indeed in two waj^s: 



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

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

 in colloidal solution. 3 As the solutions thus obtained are often very unstable, 

 it has been found advisable to help their stability by the addition of (organic 

 colloids (gelatin). We will discuss the mode of action of these so-called pro- 

 tective colloids on page 23. 



2. BREDIG 4 has discovered a method which makes possible the production 

 of pure metallic sols by the cathode spraying of metallic wires under water. 

 SVEDBERG 5 prevents the heating of the fluid in this spraying by using the 

 induction current. This makes the spraying also possible under organic fluids 

 and sols of the light metals have also been prepared. Practically sols of all 

 metals and metalloids can be prepared in this way. 



Among those bodies w r hich can be obtained in the colloidal state 

 we have acids as well as bases, and the chemical elements are also known 

 as colloids, as well as bodies of more complex molecular structure like 

 the proteins and starches. The colloid bodies, therefore, have from a 

 chemical standpoint nothing in common. More likely the colloidal con- 

 dition is due to physical properties, and this follows from the researches 

 of GRAHAM. The crystalloids and the colloids are therefore not to 

 be considered as chemically different classes of bodies, but rather only 

 as different physical conditions of matter and the boundary between 



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



2 Spring, Ber. d. d'chem. Gesellsch., 16, 1142 (1883). 



3 Miiller, Allg. Cheinie d. Kolloide. Leipzig (1907), 6. 



4 AWganische Fermente. Leipzig (1901), 24. 



5 Ber. d. d. chem. Gesellsch., 38, 3616 (1905); 39, 1705 (1906). 



