4 MICRO-ANALYTICAL METHODS 



importance— assayable vegetable drugs, all prepared food substances with chemicals 

 in solution, compound powders, pills, tablets. 



4. Substances to which the microscopical examination is not generally applied 

 — chemicals, liquids in which the insoluble particles are slight in amount, as wines, 

 brandies, comparatively pure solutions, etc. Here the centrifuge plays an im- 

 portant part. 



5. Substances in which the bacterial testing is of prime importance — milk, 

 sewage or otherwise organically contaminated water supplies, and other liquids, 

 beers, etc., contaminated foods generally. In this class of substances the micro- 

 scopical and chemical examinations become necessary in addition to the bacterio- 

 logical; in fact, a bacteriological test is incomplete without the use of a good com- 

 pound microscope. 



The work of the micro-analyst is, so to speak, on trial. The 

 doubt in the minds of the critics is due, very largely, to the un- 

 satisfactory results traceable to the efforts of those who are not 

 sufficiently qualified. Even the most skillful analysts admit 

 numerous defects in methods and shortcomings in results. For 

 example, the quantitative estimates based upon optical judg- 

 ment are approximate only, and with most workers there is a 

 very marked tendency to make these estimates volumetric rather 

 than gravimetric. This can in a measure be corrected by bring- 

 ing into play the judgment of the relative weights of the several 

 substances under comparison. For example, the amount of 

 sand present in powdered belladonna root may be volumetrically 

 estimated at 20 per cent. In this case the acid insoluble ash 

 residue may show 35 to 40 per cent, of silica. An example like 

 this also indicates why the micro-analyst should make the sand 

 and ash determinations. The percentage estimates based upon 

 microscopical examination may vary within 25 to 50 per cent, 

 when the amounts of the admixtures are small or slight. For 

 example, the actual amount of arrow-root starch in the so-called 

 arrowroot biscuit is 2.5 per cent. The micro-analyst's estimates 

 may range from a trace or small amount to 5 per cent. When 

 the quantities of admixtures are large, from 30 to 90 per cent., 

 the estimations may approximate within 10 or 15 per cent, of 

 the actual amount present. These estimates can no doubt be 



