152 THE QUANTITATIVE METHOD IN BIOLOGY 



prepared in this way, we determine exactly the weight of the 



precipitate in each 

 mixture. The value 

 of this weight de- 

 pends on two causes, 

 one of which (pipette 



A) is invariable, the 

 second one (pipette 



B) consisting of an 

 invariable cause ± a 

 variable cause which 

 is chance. The varia- 

 tion (errors) of the 

 weight precip. is 

 therefore governed 

 by the rules of 

 chance, but here 

 the effects of chance 

 are of a peculiar 

 kind. 



Suppose that the 

 greatest errors (with 

 regard to the volume) 

 committed by using 

 pipette B have been 

 + o-i6 and -oi6 

 c.c. Between these 

 extremes all the 

 errors are distributed 

 according to a curve 

 similar to Fig. i8, 

 the negative errors 

 being equal in num- 

 ber to the positive 

 ones. 



Since the volume 

 of a is exactly lo c.c. 

 in all the mixtures, 

 each negative error 

 results in a diminu- 

 tion of the weight 

 frecip. For instance, 

 the greatest nega- 

 tive error, which 

 amounts to o-oi6 of 

 10 c.c, corresponds to 984 mgr. of precipitate instead of 1000, 

 etc. The positive errors, on the contrary, don't produce any 



Fig. 19— ABCDEFGHIJK, curve of committed errors 

 (symmetrical). ABCDEL, curve of observed effects 

 (unilateral) 



