208 



ELEMENTARY CHEMICAL MICROSCOPY 



proper places, and a line is drawn through the zero and the three 

 points; the "plot" obtained will be substantially a straight line if 

 the work has been properly done. If the points laid out show 

 a marked deviation from a straight line the components differ 

 materially in their densities, or an error has been made. There 

 is thus obtained a device, Fig. 112, by which we can determine, 

 from a count of the foreign particles in any similar mixture, the 

 per cent of this foreign matter present in material of unknown 

 percentage composition. 1 



FIG. 113. 



FIG. 114. 

 Net Ruled Eyepiece Micrometers. 



FIG. 115. 



To facilitate counting an eyepiece with net micrometer is 

 essential. Rulings are usually of two types, as shown in Figs. 

 113 and 114. Where type 113 is employed the entire field of 

 view may be counted but in type 114 it is better to call a "field" 

 that area comprised within the ruled square. This system is 

 preferable to that of employing a cell with ruled bottom referred 

 to below. An attachable mechanical stage will be found to be 

 a great help in avoiding the making of counts in the same area 

 more than once. 



Although the method just described appears at first sight to 

 be crude and unreliable it has been found after a number of 

 years' trial in the hands of a large number of students to yield 

 excellent results. 



In the case of starch mixtures, where the foreign component 



1 Chamot, Seventh International Congress Applied Chemistry, Section VIIIc 

 (1909), 249. 



