THE NEW YORK JOURNAL OF PHARMACY 



Coarse Suspension y. '; Colloid 



(i) Method of dispersion: We start 

 with a solid or coarse suspension. 



(a) Mechanical grinding — Colloidal 

 osmium. 



(b) Action of light— colloidal silver. 



(c) Action of electric current — col- 

 loidal gold, etc. 



(2) Method of concentration: We start 

 with one or more colecular solutions 

 and obtain the colloid by chemical 

 reaction. Colloidal silver, gold, 

 barium sulphate, etc. 



In using these methods, we obtain at 

 times particles of different sizes , we re- 

 move the coarse ones by filtration and the 

 molecular ones by dialysis. 



Most metals have been obtained in col- 

 loidal solutions, either in water or in or- 

 ganic solvents, such as alcohol, pentane, 

 ethyl-ether or iso-butyl alcohol. Even 

 the radio-active elements, or their salts, 

 have recently (see Kolloid Zeitschrift, 

 Feb., 1914) been prepared in this con- 

 dition. 



// 



Molecular Sol. 



The Properties of Colloids, (i) Me- 

 chanical properties. They exhibit Brown- 

 ian movement when obserbed under the 

 ultra-microscope. This is a dancing, 

 trembling movement discovered by the 

 English botanist Brown in 1827. The 

 motion is back and forth from a fixed 

 central point. Smaller particles move 

 faster than large ones, particles larger 

 than ys^ i" diameter do not show the 

 motion any more. There are several 

 theories for the cause of this motion; 

 one is that the gravity of the particles is 

 'only partially overcome by the viscosity 

 of the medium, that the particles fall or 

 ris€ until the viscosity of the medium 



overcomes the-' gravity and the particles 

 are pushed back again. In coarser sus- 

 pensions, the gravity overcomes the vis- 

 cosity and a separation takes place, viz., 

 cream, in which the particles of butter- 

 fat are 2-10/^ in diameter. 



(2) Diffusion: Colloids have been 

 found to diffuse. They differ in this re- 

 spect from crystalloids only in the rapid- 

 ity of diffusion. 



(3) Osmotic Pressure. The osmotic 

 pressure of colloidal solutions is, as a 

 rule, very slight ; some have none at all. 



(4) Gravity : The density of colloidal 

 solutions cannot be calculated from the 

 density of the dispersoid and that of the 

 medium. Dispersoids show, as a rule, a 

 volume contraction and the density in- 

 creases with increasing dispersion. J. 

 Rose determined the following figures 

 for gold: 

 Gold, melted and pressed.. 19.33 



Gold, by oxalic acid i949 



Gold, by FeSOj — f i n e s t 



powder 19.55-20.71 



(5) Optical properties: Most col- 

 loidal solutions appear to be clear and 

 transparent when viewed with the naked 

 eye. Often, a turbidity, fluorescence or 

 opalescence can be noticed, especially in 

 reflected light. A ray of strong light 

 sent through a solution of this kind, will 

 show the "Tyndall phenomenon," viz., 

 the light is repeatedly reflected and po- 

 larized. The ultra-microscope is con- 

 structed so that all rays, except those re- 

 flected from the suspended particles, are 

 excluded. We see the "halo," so to say, 

 of the particles. The color of the col- 

 loidal solution varies with the degree of 

 dispersion. System Liquid-liquid most 



