15 



ir— THE EEDUCTION OF OSMIC ACID BY LIPOIDS. 

 By J. K. Partington and D. B. Huntingford. 



(Read November 17, 1920.) 



The use of osmic acid in microscopy for staining fats and allied 

 substances has long been applied. The material is immersed in a 

 dilute aqueous solution of osmic acid ; reduction of the latter 

 occurs, and a deep black substance is deposited. In the case of 

 olein globules the black stain penetrates the drop completely, and 

 extrudes on treatment with turpentine. The composition of the 

 black substance has been variously stated to be metallic osmium (i),* 

 a lower oxide Q2), or a lower hydroxide, such as Os(OH)4. At the 

 suo-crestion of Dr. J. B. Gatenby the authors decided to attempt an 

 analysis of a small amount of the material placed at their disposal. 

 This had been prepared in the following way : — Pieces of tissue 

 were placed in a clear 2 p.c. aqueous solution of osmic acid (osmium 

 tetroxide, OSO4). After a few days' use the solution became black. 

 The liquid could be filtered without losing its colour, so that the 

 substance is in the state of colloidal solution. On addition of a 

 little chromic acid, or potassium dichromate solution, a fine black 

 powder deposited, which could be filtered off and washed without 

 passing back into colloidal solution. The filtered solution some- 

 times contains a little unchanged osmic acid, according to the 

 completeness of reduction of the solution. In the particular solu- 

 tion used, a little osmic acid was still present before washing. 

 The black precipitate was dried on the filter paper in a desiccator 

 over sulphuric acid for ten days. A slight greenish tinge possessed 

 by the moist substance disappeared on drying, and the product 

 was a pure black powder, easily detached from the paper. The 

 powder was not heated during the drying process. 



For various reasons it was assumed that the material so 

 obtained was the same as that present in the stained fat globules, 

 but further experiments with osmic acid and pure olein in vitro 

 are in progress which will decide this point conclusively. 



From its method of preparation the material might contain the 

 following substances : Metallic osmium ; lower oxides of osmium ; 



* The italic figures within brackets refer to the Bibliography at end of the 

 paper. 



