464 PROCEEDINGS OF THE AMERICAN ACADEMY, 



The work described in the followiug sections shows that it is very 

 difficult to determine the degree of activity of particular factors, even 

 when such factors have been purposely reduced to the smallest number 

 that the vital conditions of the animal permit. In the preceding experi- 

 ments we have an unknown mixture of both salts and food, dead as well 

 as living, with an excess of one salt. In view of the control experiment, 

 differing only in the absence of the excess contained in the test, any 

 difference in result can be ascribed undoubtedly to that excess. But in 

 view of the further fact, developed in subsequent experiments, that the 

 simultaneous presence of several substances modifies their individual 

 activity, it is difficult to see how any satisfactory insight into the meaning 

 of the above processes can be obtained from the use of mixtures of large 

 and unknown complexity. This consideration shows that there is a limit 

 to the value of the method previously employed, and also suggests a 

 better plan of experimentation. 



"With the above reservations, I attribute to potassic chloride a degree 

 of specific action not very commonly observed among salts, but probably 

 of more frequent occurrence than our present means of observation indi- 

 cate. The acceleration due to chloroform is not at all different from 

 that common to nearly all poisons used in appropriately low concentra- 

 tions. In both the above cases there is a reaction-chain having a stimu- 

 lating substance at one end of the series and the division-reaction at the 

 other. Except by mere speculation, I am unable to interpolate other 

 elements into the chain. 



The action of a temporarily applied substance seems to me to depend 

 much on the greater or less permeability of the cells to the substances 

 applied. Either enough of the permeating substance was retained by 

 the organism to keep up the effect, or the modification at first produced 

 by it was strong enough to affect the activities of the organism after the 

 removal of the reagent. To the subject of permeability in general I shall 

 frequently return in later sections. 



I 



V. Observations on Single Salts and Water. 



I next planned a series of experiments in which the object was to 

 compare the effects of a number of physiological salts upon the division- 

 reaction when applied singly and in the higher ranges of concentration. 

 The number of salts so experimented with was for several reasons small. 

 First, the time required for this and subsequent work was much more 

 than at first had seemed probable. The precautions taken in making the 



