MICROCHEMISTRY. 147 
is to the organic laboratory what spectrum analysis is 
to the inorganic.” When it is necessary to separate 
two closely related isomers, to determine the identity: 
of two substances prepared by different methods, to 
distinguish differences of elementary composition from 
those due to allotropism, or to detect minute admixtures 
of an impurity, it frequently occurs that purely chemical 
methods are tedious and complex, while microscopical 
examination furnishes a rapid and easy solution. In 
general the method of study consists in allowing the 
substance in question to crystallize in contact with a body 
of known composition, with which it is to be compared. 
This may be done in three ways—by adding to one 
substance, in a melted condition or in solution, a crystal 
of the other, by preparing a mixture of both in a liquid 
condition and allowing the mixture to cool, and best; 
perhaps, by crystallizing from films of the two sub- 
stances brought just in contact with each other. 
When solid substances are to be compared, a small 
portion of one is placed under a cover-slip and heated 
till it melts, the amount of material being so small as 
not to reach the edge of the cover. A grain of the second 
substance is then placed just outside the cover-slip, and 
in turn melted so as to flow under and join the first along 
a line which can be observed under the microscope. (In- 
cidentally it may be noted that by heating two bodies 
simultaneously under the microscope a good measure is 
obtained of slight differences in melting-points. Thus 
the similar crystals of dinitrobenzol and dinitrotoluol 
may be easily distinguished.) As two substances cool 
and recrystallize, the phenomena along the contact zone 
