366 



EFFECT OF DENSITY OF THE MEDIUM [Cn. XIII 



mined by counting. The results of some of these countings are 

 given below for a number of salts : 



TABLE XL 



AVERAGE INCREASE Per Cent OF INDIVIDUALS OF DERO VAGA REPRODUCING, 

 DURING TEN DAYS, IN SOLUTIONS OF DIFFERENT SALTS AT VARYING CON- 

 CENTRATIONS. MINUS QUANTITIES INDICATE DIMINUTION IN NUMBER OF 

 INDIVIDUALS 



These columns, and especially the first one, show a close 

 relation between concentration and growth (as tested by mul- 

 tiplication of individuals). They show also that the diminished 

 growth falls off rapidly at first with slight increments of con- 

 centration, then less slowly at the higher grades (Fig. 99). 

 Finally they show that different salts have diverse osmotic 

 effects, for sodium chloride is less retarding than any other 

 salt at the same percentage of concentration. The effect of 

 the remaining salts is seen to increase as the molecular weight 

 diminishes, and therefore the osmotic effect increases (Fig. 99). 

 The fact that magnesium sulphate dissociates at these weak 

 concentrations only about two-thirds as much as calcium chlo- 

 ride does, would lead us to expect even a relatively smaller 

 effect as compared with calcium chloride than we find (see 

 p. 74, note) ; perhaps further experimentation would give facts 



