MEASUREMENT OF VARIABLE PROPERTIES 151 



successive figures varying gradually. In the last example 

 (acid + alkali, § iii), on the contrary, discontinuous variation 

 exists in the effects of chance ; there is, indeed, a distinct gap 

 between red and blue. 



The above phenomena recall respectively the continuous 

 and discontinuous variation observed in animals and plants.^ 

 (See § 21.) 



A comparison between the fourteenth and the fifteenth 

 examples is interesting for the biologist. In both cases 

 (volume and colour) all the conditions are the same, the opera- 

 tions are the same, the causes are the same. The effects, how- 

 ever, are quite different : continuous in one case, discontinuous 

 in the other. It might even be possible to observe both effects 

 (volume and colour) at the same time in each portion (speci- 

 men), if it is supposed that the chemical reaction has no 

 influence upon the total volume. 



This being admitted, each portion acid + alkali may be 

 looked upon as being an individual of a certain species of which 

 we have investigated two primordia, volume and colour. With 

 regard to the volume, continuous variation is observed, the 

 variation curve being regular, as in Fig. i8 ; with regard to the 

 colour, variation is discontinuous. We might be tempted to 

 ascribe the difference to a difference in the causes. ^ We know, 

 however, that the very same causes have produced dissimilar 

 effects because each property has its own specific energy 

 which produces its characteristic states of equilibrium. ^ (Com- 

 pare Centaurea, § 47, p. 58.) 



§ 113.— SIXTEENTH EXAMPLE : UNILATERAL VARI- 

 ATION (UNILATERAL VARIATION CURVE).— We dispose 

 of two watery solutions containing respectively two salts a 

 and h. The solutions are prepared in such a way that 10 c.c. 

 of a precipitate exactly 10 c.c. of h, the weight of the precipitate 

 being i gr. = 1000 mgr. 



By means of pipette A (§ no) we measure exactly 10 c.c. of a ; 

 by means of pipette B (which is imperfect) we measure 10 c.c. 

 of h. Both volumes are mixed. A number of mixtures being 



unit. On p. 145 the unit is o'oi of the density of water. If we take, for 

 instance, o'ooi as unit, the difference under consideration is ten times smaller 

 than when 00 1 is taken, etc. However small the adopted unit may be, perfect 

 continuity is never obtained, owing to the necessity of using ciphers for the 

 expression of the measurements. 



^ The form of variation observed in the last example (red-blue) is often called 

 alternative variation. It may be looked upon as being discontinuous variation 

 with two terms. 



2 For instance, to the existence of different determiners, factors, pangenes, etc. 



^ The same cause has produced two kinds of variation, in one case con- 

 tinuous, in the other case discontinuous ; but there is a connection between 

 the two kinds — e.g. if a is a blue specimen and /3 a red, then vol. oj a > 

 vol. oj /3 and this rule is invariable. 



