QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 209 
unit.” The eyepiece micrometer is ruled and so adjusted that 
with a given objective and eyepiece the smallest squares are 
equal to a standard unit, Fig. 126. 
Method 3. — When isolated particles of sufficiently definite 
shape can be found and are of known composition and density, 
it is possible to calculate their weight from micrometric measure¬ 
ments. 
This method is especially useful in estimating the weight of 
substances imbedded in other materials in such a way as to be 
not easily separated; in the determination of poisons in forensic 
investigations; and in determining the weight of tiny metallic 
beads or pieces of metal, which, for one reason or another, can¬ 
not be weighed on a balance. 
The dimensions of the particles are first determined by any 
one of the micrometric methods described in Chapter VII. 
From these measurements the volumes of the particles are 
calculated and their weight then obtained by multiplying by 
the specific gravity of the substance. 
If the substance whose weight is to be determined can be 
made to take the form of a sphere the data found are usually 
as accurate as those obtained by weighing, but it is obvious that 
if only more or less irregular particles or crystals are available 
the method should be regarded as giving merely approximate 
results. Even so, the method must be recognized as of value 
since in many instances no other system of solving the problem 
of percentage composition may be available. 
This method of quantitative analysis by means of the micro¬ 
scope is very old and has been successfully applied to the deter¬ 
mination of gold and silver in fire assays (especially with the 
blowpipe) where the metallic beads obtained on cupellation 
are too small to weigh even upon a sensitive assay balance. 
With carefully fused beads it has been shown^ that the results 
are accurate and quickly obtained. 
The first essential is that the little metallic globule shall be 
a perfect sphere. If it is not, it is placed in a tiny cavity in a 
piece of charcoal and fused before the blowpipe; after cooling, 
* Goldschmidt, Zeit. anal. Chem., 16 (1877), 434. 
