172 Transactions of the Society. 



Ten fields from the second slide gave counts as follows : — 



Lycopodium spores . . 11, 6, 5, 8, 12, 5, 8, 8, 14, 6 =83 



Maize starch grains . . 30, 28, 34, 46, 26, 17, 18, 11, 44, 40 = 294 



Giving 354 maize starch grains for 100 lycopodium spores. 



The average for the twenty fields is 361 maize starch grains for 

 100 lycopodium spores. 



Hence a certain weight of the 50 p.c. mixture contains 475 

 maize starch grains, and an equal weight of the adulterated flour 

 contains 361 maize starch grains. Therefore the amount of maize 

 starch in the flour is 50 X 361-4- 475 = 38 p.c. The actual 

 amount of maize starch present in the flour was 37 * 8 p.c. 



Eemarks. 



The flour and lycopodium are mixed by rubbing them together 

 by means of a flexible steel spatula upon a glass plate or porcelain 

 slab. The mucilage of tragacanth, which is prepared by mixing 

 1 • 25 grm. of powdered gum tragacanth with 2 • 5 c.c. of alcohol 

 (90 p.c), and adding 100 c.c. of distilled water as rapidly as 

 possible and shaking vigorously, should be made some hours 

 before it is needed for use so that the gum may be fully swollen. 

 This suspending agent is best added a little at a time and 

 thoroughly incorporated with the powder on the glass plate by 

 trituration with the spatula. The mixture is transferred to a 

 corked or stoppered tube, and the plate is cleaned by adding more 

 mucilage in small quantities at a time, mixing it with any residue 

 and transferring the successive quantities to the tube until none 

 of the powder remains on the plate. The contents of the tube are 

 thoroughly shaken up and a drop is quickly removed with a glass 

 rod and mounted for examination. 



The fields in which the counts are made are selected so as to 

 include some from all parts of the preparation. This is necessary 

 because the mount may be thinner on one side than on another, 

 thus resulting in a slight unevenness of distribution. It is also 

 necessary to avoid counting the same field twice, hence the positions 

 of the fields are fixed beforehand by choosing such as are at certain 

 measured distances from the centre of the cover-glass. The slide 

 is placed on the stage of the microscope so that the centre of the 

 cover-glass is immediately beneath the front lens of the objective, 

 and the position of tlie right-hand near corner is read off on the 

 two graduated scales of the mechanical stage. The slide is then 

 moved by means of the milled heads which actuate the mechanical 

 stage until each specified field is brought in succession under the 

 microscope objective. The positions suggested for the fields are 



