169 



VI. — The Lyco^odium Method of Quantitative Microscopy, 

 By T. E. Wallis, B.Sc. (Lond.), FJ.C. 



{Read March 17, 1920). 

 One Text-Figure. 



The use of the microscope for quantitative measurements is 

 attended by many difficulties. Such determinations are, however, 

 of great importance, since they offer^the only available method of 

 solving certain analytical problems. For example, the proportion 

 of maize starch added to the ordinary wheat flour cannot be deter- 

 mined by chemical methods, which will give accurately the total 

 starch present, but fail to differentiate between different starches. 

 Similarly one can determine chemically the total woody structures 

 (crude fibre) in a powder-like pepper or gentian root, but cannot 

 tell what proportion consists of foreign stone cells, if such are 

 present. Both these problems can be satisfactorily solved by 

 microscopical methods. 



In carrying out work of this kind attention must be given to 

 such thorough mixing of the materials as will assure efficient 

 sampling. The necessity for care in this particular is evident 

 from the fact that the result is based upon observations made of 

 the composition of a very minute quantity of powder — namely, 

 that which occupies about twenty fields of view. Assuming that 

 there is about 0*2 grm. of substance in 10 to 20 c.c. of the 

 fluid used as a suspending agent, and that one is working with a 

 one-sixth inch objective, then under ordinary circumstances twenty 

 fields will represent from one-250th to one-lOOth part of a milli- 

 gramme of powder. The mixing then must be so thorough that 

 every 200th part or thereabouts of a milligramme of the mixture 

 shall have a composition similar to that of the whole sample. 



The suspending agent, too, must be of a suitable character. It 

 should not bring about a separation of the constituents, and should 

 not allow the suspended material either to sink or to rise too 

 rapidly. It should also contain no structures that could be mis- 

 taken for any that are present in the powder under investigation. 



Further, the materials for the determination of which quanti- 

 tative methods are needed are of very varied types, ranging from 

 substances like starches, whose structure is quite simple, to powders 

 of increasing complexity, such as olive stones, mixed flours, pepper, 

 insect flowers, etc. 



