320 E. T. Bell, 



It seems clear that the blackening or browning of liposomes in 

 osmic acid is not due at all to palmitic or stearic fat. It has already been 

 pointed out that the crystals of palmitic and stearic acids are not stained 

 by scarlet red or Sudan III. These fats, therefore do not form any part 

 of the stainable substance in the liposomes. 



Altmann found that lecithin is colored macroscopically black but 

 microscopically a light grey by osmic acid. Wlassak [1898] agrees 

 essentially with Altmann. Microscopically he obtained a yellowish- 

 brown color which changes to pure grey if left in water that is changed 

 repeatedly. Wlassak states nevertheless that we are justified in attri- 

 buting the blackening of medullary substances with osmic acid to the 

 lecithin present, but his arguments for this point are not convincing. 

 It can hardly be said that we know the exact chemical body in the medul- 

 lary substance which reduces osmic acid. Aschoff [1909] believes that 

 it is practically impossible to obtain pure lecithin, and if this view 

 be correct the grey color obtained with osmic acid may be due to a 

 trace of olein. 



My own observations on the staining of lecithin with osmic acid 

 are in agreement with those of Altmann and Wlassak. But we cannot 

 decide whether pure lecithin reduces osmic acid until we are assured 

 that the lecithin used is free from olein. 



It has been known for a long time that osmic acid may be reduced 

 by substances other than fat. It is known that several of the more 

 readily oxidizable metals will reduce osmium from aqueous solutions. 

 Azoulay (1895) states that osmic acid gives a black, brown-black, or violet 

 blue (according to the quality and quantity of the substance present) 

 with gall-nut, tannin, gallic and pyrogallic acids, hydroquinone, etc. 

 Neubauer (1902) beheves that osmic acid is not a reagent for fats 

 but for substances having a double bond between the C- or CH-atoms. 



I have verified Azoulay's observations with gallic and pyrogallic 

 acids, and hydroquinone. The last named substance produces a blackish 

 color, the two acids a violet-blue color immediately upon being brought 

 in contact with osmic acid. It is however improbable that any of these 

 substances are present in the liposomes. At the present time oleic acid 

 and triolein are the only fats that are certainly known to reduce 



