WHAT ARE COLLOIDS 11 



in organic fluids especially in isobutyl alcohol. R. LORENZ has even 

 made metal sols in red hot solution (pyrosols) by electrolysis of 

 molten lead and cadmium salts, etc. To distinguish them from the 

 water soluble hydrosols and from the hydrogels we call them organo- 

 sols or organogels and according to the solvent as alcosols, etc. These 

 do not occur in nature and are therefore of no importance to us. 



An investigation of such colloids as have /ate, lecithin and cholesterin 

 either as a dispersing medium or as a "dispersed phase" (cf. p. 12) 

 would certainly be of great importance to biology and medicine. 

 The fact that fats and oils, especially mineral oils, may serve as a 

 dispersing medium for colloids has been mentioned by D. HOLDE * 

 in his work on the "physical condition of solid fats" and this was 

 confirmed by investigations of H. BECHHOLD, 1 who was able upon 

 ultrafiltration (through a toluol-glacial acetic acid collodion filter) 

 to hold back from a crude oil a part of the asphalt colloidally dis- 

 solved in it. Recently C. AMBEEGER dissolved a series of metals 

 (gold, silver, platinum, arsenic, etc.) in lanolin. Some of these 

 solutions have therapeutic application. 



By ultrafiltration H. BECHHOLD separated from commercial 

 chlorophyl the coloring matter and the wax-like products which were 

 evidently held in colloidal solution. 



Protective colloids also exist for organic fluids. Iron oxid gel and 

 iron oxid hydrosol, rennin and trypsin, as well as albumoses which 

 are completely insoluble in chloroform, become soluble in it with the 

 aid of lecithin acting as a protective colloid. 



Thus far we have sought to obtain a picture of what we term 

 "colloids" and now we shall strive to elucidate upon what their 

 properties depend. In solutions of colloids and in gels, we have 

 mixtures of solids or of fluids with fluids. It has for a long time 

 been known, that at the interface between two substances which do 

 not mix (air and water, oil and water, glass and water) there occur 

 phenomena, called surface phenomena. For instance, the surface of 

 water in contact with air acts as a pellicle; if we allow water to drip, 

 each drop reaches a considerable size before its weight breaks through 

 the surface skin or pellicle and the drop falls. This surface skin is 

 much weaker in the case of alcohol, so that drops of alcohol falling 

 from the same tube are much smaller than those of water. The fol- 

 lowing is another example of a surface phenomenon: oil forms a 

 sphere in a suitable mixture of water and alcohol; if we raise the 

 specific gravity of the water by removing some of the alcohol, the 

 oil rises and spreads out over the surface of the water. 



Such surface phenomena are very numerous; they are brought 

 1 Unpublished. 



