204 ELEMENTARY CHEMICAL MICROSCOPY 



as starches, flour, meals, spices, etc., powdered sucrose, dextrose, 

 lactose or soluble dextrin are most useful as diluents. 



When the mixtures under examination are of a density only 

 very slightly greater than water and are insoluble therein, and 

 therefore if suspended would subside only after a long period, it 

 is possible to weigh out a portion of the mixture, add it to water, 

 or better, water and glycerine, in a small graduated flask, fill to 

 the mark, shake well and quickly remove one cubic centimeter 

 or less, for counting. This method avoids the error arising from 

 non-uniform quantities, but is longer and more cumbersome than 

 the methods already described. 



To further guard against the rapid subsidence of the particles 

 in suspension, gums, dextrin, gelatine or mucilage may be added 

 to the glycerine- water mixture; in most cases this will be found 

 to be a decided improvement. 1 When the removal of fats in 

 no way alters the morphology nor the dimensions of the elements 

 of the powdered material it will generally be found to be an 

 advantage to extract the sample with ether or petroleum ether 

 after drying and weighing. The particles of the powder are 

 more easily and uniformly " wetted " and are therefore more 

 readily suspended throughout the liquid and may also be more 

 evenly distributed upon the slide. 



In order to obtain greater accuracy than is possible by the 

 methods already described Wallis 2 mixes with a known weight 

 of the powdered substance, a known weight of Lycopodium; 

 suspends the mixture in gum tragacanth-glycerine mixture or 

 in the case of fatty powders in oils : the number of characteristic 

 elements and the number of Lycopodium spores per field on a 

 microscope slide are counted. The method in brief is as follows; 

 for details the reader is referred to the original article. 0.2 gram 

 of a mixture of known percentage composition is thoroughly 

 mixed with 0.1 gram of Lycopodium and suspended in 20 c.c. 



1 Schneider (Microbiology and Microanalysis of Foods) finds gum acacia most 

 useful. The addition of a 3 per cent solution to the glycerine-water (1 : 1) medium 

 is recommended, in the proportion of 15 c.c. of the gum solution to 10 c.c. of the 

 glycerine mixture for a 5 grams sample of the powdered material. 



2 Analyst; 41, (1916), 357. 



