2 BULLETIN 1108, U. S. DEPARTMENT OF AGRICULTURE. 
lation of the original composition of the materials from the analytical 
figures would be practically out of the question in all but a relatively 
few and simple cases. 
Even where chemical analysis, either qualitative or quantitative, 
gives the desired information as to the existence of a certain com- 
pound, the time consumed in making the analysis is comparatively 
long. A method which would give reliable and rapid identifications 
is obviously desirable and for certain work imperative. For instance, 
a chemical analysis would readily give the percentage of the various 
elements in a mixed fertilizer, but further data as to the actual com- 
pounds of potash, say, which are present in the sample might be 
necessary for certain plot or pot experiments. 
A number of methods designed to furnish evidence as to the con- 
stituents of mixtures have been proposed and used in certain limited 
fields of study. The separation of minerals in rock powders according 
to their specific gravities by means of solutions of appropriate densities 
was long in vogue, and even now is used occasionally. Ordinary 
microscopical observation of crystal forms has been used extensively, 
and, though sometimes abused, in the hands of an expert it can be 
made of considerable value. Its application, however, is limited to 
well developed crystals, and even then it furnishes only meager data 
of a definitive kind. The danger attending its use becomes apparent 
when one tries to identify one of the numerous “needle-shaped”’ 
crystals with no more data than can be obtained with the cross hairs 
and possibly the rotating stage of the microscope. Microchemistry 
at its best gives scarcely better results than a qualitative chemical 
analysis. The observation of absorption spectra with a micro- 
spectroscope is of extremely limited application. Up to the present 
time, petrographic methods are the best and apparently the only 
general solution of the problem. 
PETROGRAPHIC METHODS. 
The possible range of application of petrographic methods to chem- 
ical problems is almost unlimited. They have been successfully 
applied to the identification of compounds formed in concrete and 
cement mixtures and to general ceramic problems. It has been 
found possible to differentiate between phrenosin and kerosin derived 
from both the brain and the soil. The alkaloids and sugars afford 
notable instances of the rapid identification of substances without 
tedious chemical procedures... Most phase rule work presents 
problems easily solved by the petrographic microscope. This appli- 
cation has already found commercial use in the study of brines. In 
the laboratory of the Bureau of Soils it has been shown to be feasible 
1 Wherry, E. T., U.S. Dept. Agr., Bul. 679 (1918). 
