206 ELEMENTARY CHEMICAL MICROSCOPY 



characteristic in appearance yet differing according to the per- 

 centage composition, the recognition of these crystalline phases 

 will serve to indicate the probable composition of the mass. 



The last method (3) is restricted to materials such as alloys 

 or related substances. An expert, knowing the characteristic 

 appearance following certain treatments, is able, on studying 

 materials of known components but of unknown percentage, to 

 decide upon the probable proportion of the chief constituents 

 without the necessity of a quantitative analysis. This type of 

 analysis by means of the microscope can be practiced only by 

 experts after long study and investigation and cannot therefore 

 be here discussed. 



The first method may be employed in the quantitative analy- 

 sis of all mixtures consisting of individual particles, fragments or 

 crystals, which are not too large for microscopic examination, 

 providing the component particles differ sufficiently in appear- 

 ance to permit of identification and that mixtures of known per- 

 centage composition can be prepared in the laboratory. Since 

 this method has its chief application in estimating the amount 

 of adulteration in a substance, the discussion will be confined to 

 this aspect only. 



Method I. Prepare three standard mixtures containing the 

 same components as the commercial products to be examined. 

 In preparing these standards the adulterant must be carefully 

 weighed out and added to a definite weight of the pure product; 

 after thorough mixing, three mixtures of known per cent of adul- 

 teration are thus obtained. 



From each one of these standards in turn, several like portions 

 are taken, placed upon glass object slides in a drop or two of 

 suitable medium (usually glycerine and water i : i), 1 distributed 

 uniformly in the mounting medium and covered with a square 

 cover glass, care being taken to avoid air bubbles; use just 

 sufficient mounting medium to ensure an even distribution of the 

 material throughout the whole area covered by the cover glass 



1 Smith, Health Mag., 5 (1898), 286, has shown that in the case of starch 

 mixtures a mounting medium of equal parts of glycerine, water and 50 per cent 

 acetic acid is preferable. 



