( 704 ) 



After a preliminary experiment had proved that tlie toxic limit 

 of Cu SO4 for potato lay below a concentration of 0,005 granmiol. 

 per litre, four fragments of potato were dried with filtering-papei", 

 weighed, and placed in solutions of Cu SO4 containing: 



a 0,001 ; h 0,002 ; c 0,003 and d 0,004 gr. mol. 



The bits of potato weighed respectively : 



a 3,775 ; h 3,225 ; c 2,860 and d 3,195 gr. ; 



After having stayed in the solutions for 24 hours, they were 

 dried and weighed again, the results being : 



a 4,620; h 3,310; c 2,895 and d 3,260 gr. 



So they all had absorbed water; the toxic effect of the cupric 

 sulphate penetrating at the same time would now soon become 

 apparent, however. The bits were washed and placed in water from 

 the supply (water from the dunes); after 24 hours they weighed: 

 a. 4,670; h. 3,350; c. 2,825 and d. 3,150 gr. 



This time c. and d. had lost weight and this loss increased steadily 

 during the following day, whereas a and b went on absorbing water. 

 The toxic limit of Cu SO4 for bits of potato weighing 3 — 5 grammes 

 consequently lies, after 24 hours, between 0,002 and 0,003 grammol. 

 per litre, i.e. between 0,03 and 0,05 per cent (molecular weight of 

 Cu SO, = 159). 



Henceforth a piece of tissue was considered undamaged if, after 

 having stayed in the poisonous solution for 24 hours and then for 

 another 48 hours in water (once or twice renewed), it had, at all 

 events, not lost weight, if it had not gained. It is obvious that in 

 these experiments only such organs can be used as will remain alive 

 for a fairly long time, when immersed in water. I can state concerning 

 potatoes, that normal fragments, placed in water which was daily 

 renewed, even after 18 — 20 days did not lose weight but absorbed 

 small quantities of water. It made no difference, at least within this 

 period of time, whether water from the supply or distilled water 

 was used. In all similar experiments the results obtained by weighing 

 are confirmed in a striking manner by the circumstance that bits of 

 potato, when they die oft', turn dark-grej^ (conversion of tyrosine 

 into homogentisinic acid by enzyme-action). Also various other parts 

 of plants show some similar phenomenon which may serve as a 

 check, in the first place the diffusion of colouring matter, as with 

 red beetroot. Begonia and others. 



In the manner described above, also the harmful limit of con- 

 centration may be determined of neutral mineral salts which in a 

 certain dilution are innocuous for a long time, but in more concen- 



