7° 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 60-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 bé 
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 
