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 1 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, 



1 Goklschmidt, Zeit. anal. Chem., 16 (1877), 434. 



