The Lycojpodium Method of Quantitative Microscopy. 171 



result should agree closely with that previously obtained. In case 

 of a disagreement greater than 10 p.c, which will rarely occur in 

 practice, fresh counts must be made. 



One next prepares a suspension of a similar weighed quantity 

 of the article to be examined, mixed with an amount of lycopodium 

 spores equal to tlie weight of spores used in the first part of the 

 experiment. Drops of this suspension are mounted, and counts of 

 twenty fields are made as in the former case. 



For both suspensions the number of characteristic particles 

 counted for 100 lycopodium spores is calculated. These two 

 numbers represent the quantities by weight of one ingredient present 

 in equal weights of the two mixtures, and since one percentage is 

 known, the other is immediately obtained by simple proportion. 



Example. 



An example will simplify the explanation. It was desired to 

 find the proportion of maize starch that had been added to some 

 ordinary wheat flour. A mixture of wheat flour and maize starch 

 in equal proportions was prepared, and * 2 grm. of this mixture 

 was carefully mixed with 0*1 grm. of lycopodium spores, and 

 about 20 c.c. of mucilage of tragacanth. A drop of the suspension 

 was mounted, and the following counts obtained in ten selected 

 fields : — 



Lycopodium spores . . 14, 15, 7, 11, 11, 5, 4, 13, 12, 10 = 102 



Maize starch grains . . 43, 74, 53, 33, 61, 39, 25, 38, 57, 59 = 482 



Giving 473 maize starch grains for 100 lycopodium spores. 



A second slide was prepared by mounting another drop of the 

 same suspension, and the counts obtained were as follows : — 



Lycopodium spores . . 11, 11, 11, 7, 12, 7, 4, 9, 9, 12 = 92 



Maize starch grains . . 61, 45, 57, 52, 63, 28, 23, 37, 47, 86 = 439 



Giving 477 maize starch grains for 100 lycopodium spores. 



It will be seen that the two sets of counts give closely similar 

 results, and the average for the 50 p.c. mixture is 475 maize starch 

 grains for 100 lycopodium spores. 



The adulterated wheat flour was next similarly examined by 

 mixing • 2 grm. of the flour with • 1 grm. of lycopodium spores, 

 and about 20 c.c. of mucilaoje of traG^acanth. 



Ten fields from the first slide gave the following counts : — 



Lycopodium spores . . 13, 10, 14, 20, 21, 11, 4, 14, 16, 12 = 135 



Maize starch grains . . 48, 88, 42, 87, 64, 25, 24, 43, 77, 49 = 497 



Giving 368 maize starch grains for 100 lycopodium spores. 



