422 



SCIENCE. 



[N. S. Vol. XIX. No. 



tures in distilled water, nor did any marked 

 result appear. 



In the case of the copper sulphate solu- 

 tions, the action of the insoluble bodies 

 seemed to be especially well marked. In 

 many instances the introduction of 40 grms. 

 of sand into 150 c.c. of a given solution 

 was seen to accelerate the growth rate from 

 a few millimeters in the check solution con- 

 taining the copper salt alone to a growth 

 rate nearly normal for the distilled water 

 used in making the solutions. The modi- 

 fying action of the insoluble substances was 

 found to vary widely, according to the 

 quantity used, assuming that the volume 

 and concentration of the solution are con- 

 stant. The following instance is typical: 



In this case growth was practically at a 

 standstill in the copper solution. The in- 

 troduction of 40 grms. of sand gave a 

 growth rate equal to one half of the growth 

 rate in distilled water. The addition of 

 further sand resulted in a growth rate of 

 increasing rapidity, until when 120 grms. 

 were added a maximum was reached. Fur- 

 ther addition of sand was followed by a de- 

 creased rate of growth, finally approxima- 

 ting that of the check. 



Unpublished investigations were cited in 

 which it was shown that a somewhat similar 

 situation results when a series of progress- 

 ively diluted solutions of the copper sul- 

 phate is tested by the above method with 

 the lupine radicles. In concentrations of 

 toxic activity, growth is quickly svippr.essed. 

 As the solution is diluted more and more, 

 the growth rate increases, until a rate char- 

 acteristic of distilled water is reached. In 

 the action of the copper sulphate solution 



the phase characterized by the depressed 

 growth rate may be termed the depression 

 phase. This is succeeded, at a given con- 

 centration, by a growth rate like that of the 

 check, the copper being neutral in its 

 action. Upon further dilution the radicles 

 are found to grow more and more rapidly 

 until at a definite concentration a maxi- 

 mum rate is reached, which much exceeds 

 that of the check. Further dilution at this 

 point is followed by a falling off in the rate 

 of growth until at a definite concentration 

 the solution is so weak in its action as to 

 leave the medium equivalent to so much 

 distilled water. The first neutral phase is 

 thus seen to be followed by a phase of pro- 

 nounced acceleration of growth rate. This 

 acceleration rises to a maximum and de- 

 clines to a second neutral phase of indefi- 

 nite extent covering all greater dilutions. 



From these experiments it seems clear 

 that the presence of insoluble substances 

 exerts an effect closely paralleling that of 

 simple dilution. By comparison with the 

 growth curve obtained in increasingly 

 diluted solutions, it is possible to indicate 

 what amount of copper remains in the free 

 solution unremoved, physiologically speak- 

 ing, by the solid. 



Various investigators have shown that 

 gases are condensed over surfaces of solid 

 bodies, the layer of gas in contact with 

 them containing a much greater concen- 

 tration of molecules than is contained by a 

 like volume of free space. This has been 

 extended to solutions. It has been shown 

 that the walls of the containing vessels or 

 solid particles placed in the solutions con- 

 dense substances from aqueous solutions. 

 It has been found that this process, known 

 as adsorption, is largely dependent upon 

 the nature of the adsorbing solid and on 

 the amount of surface it offers to the solu- 

 tion. The adsorptive activity of sand, 

 filter paper, charcoal and other bodies has 

 been demonstrated. These bodies are all 



