70 ELEMENTS OF APPLIED MICROSCOPY. 



coffee and mustard, containing a number of complex 

 tissues, is more difficult, but a careful comparison with 

 samples of known purity insures reasonable certainty. 



The technique of the microscopical examination is 

 very simple. The substance to be observed is ground up 

 fine (so as to pass through a 6o-mesh sieve), further com.- 

 minuted between the fingers, mounted in water, and 

 examined directly. It is sometimes advisable to rub the 

 powder to a still finer condition by manipulating the 

 cover-glass and slide between the thumb and finger. 

 The process should not be carried too far, because in 

 tissues which are broken up into very fine fragments the 

 characteristic structures are often indistinguishable. Bet- 

 ter views of the structure of opaque objects may be 

 obtained by clearing or rendering them transparent by 

 the action of strong alkalies, caustic soda, or chloral 

 hydrate. In practice, however, this is not generally 

 necessary. 



The student should be careful that whatever comes in 

 contact with the specimen to be examined is quite clean 

 and free from the contamination of previous specimens. 

 Several samples from different portions of the material 

 should be examined in order to gain an idea of its aver- 

 age composition. Since the recognition of fragments of 

 tissue depends largely upon the personal element, certain 

 points catching the eye of each observer, the comparison 

 with standard pure samples should never be dispensed 

 with. 



2. General Nature of Food Adulterants. — A very 

 thorough and systematic examination of foods is carried 



