COLLOIDS. 37 



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 44. 



2. BREDIG l has discovered a method which makes possible the obtainment 

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

 SVEDBERG 2 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. Sols of all metals and 

 metalloids can be prepared practically in this way. 



Among those bodies which 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 colloid con- 

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

 of GRAHAM. 



In order to give a better review we will give a classification of the 

 colloids which seems, for the present, to be rather universally accepted. 

 This was first suggested by PERRIN S and later accepted by HOBER/* 

 A. MuLLER, 5 and Wo. OsTWALD, 6 although different authors use different 

 names for the two -classes. The classifications of HARDY 7 and ZSIG- 

 MONDY 8 have also much in common with the classification given below. 



One of the two groups of colloids is called hydrophile colloids (emul- 

 sion colloids, emulsoides) because in the aqueous solution a certain rela- 

 tion still exists between the dissolved substance and the solvent which 

 is evident especially by a certain viscosity of the solution. The hydro- 

 phile colloids often gelatinize on cooling, the gel is again soluble in 

 water (reversible), and in general the hydrophile colloids are separated 

 from their solution by electrolytes with greater difficulty than the col- 

 loids of the second group. Bodies of the greatest importance for phys- 

 iological chemistry like the proteins, starch, and glycogen, belong to the 

 hydrophile colloids. 



Contrary to the hydrophile colloids the colloids of the colloidal metal 

 type are called suspension colloids (suspenoids) as they must be con- 

 sidered as suspended solid particles in a solvent and have no close 

 relation to the solvent. The viscosity of the solution does not differ 

 much from that of the pure solution; besides this, the suspension col- 



1 Anorganische Fermente. Leipzig, 1901, 24. 



2 Ber. d. d. chem. Gesellsch., 38, 3616, 1905; 39, 1705, 1906. 



3 Journ. de Chimie phy., 3, 84, 1905. 



* Physik. Chem. d. Zelle u. Gewebe, 2 Aufl., 1906, 208. 

 6 Allg. Chemie d. Kolloide, 1907, 187. 



6 Zeitschr. f. Chem. u. Ind. d. Roll., 1, 331, 1907. 



7 Proc. Roy. Soc., 66, 95, 1899. 



8 Zur Erkenntnis d. Roll., 1905, 16. 



