QUANTITATIVE SELECTIVE POWER 



121 



even more rapidly than does an equal surface area of a leaf, assimilating 

 in sunlight ; while in a drop of tannic acid, surrounded by a precipitation 

 membrane formed by the solution of gelatine in which the drop is floating, 

 methyl-blue is absorbed and accumulated, for precisely the same reason as 

 in a living cell containing tannic acid in solution in the cell-sap. 



In order to demonstrate the mode of accumulation of substances in a cell, the 

 precipitation of copper by zinc may be employed. In a glass cylinder, the ends of 

 which are covered by parchment or linen, a roll of zinc is inclosed, and after being 

 filled with water the whole is immersed in a very dilute solution of sulphate of 

 copper (Fig. 6). The appearance of zinc sulphate in the outer fluid shows that one 

 of the decomposition productions of sulphuric acid diffuses outwardly. 



On the method of using aniline dye to demonstrate passive secretion, see p. 94. 

 In a watery solution of methyl violet of the strength i : 100,000,000, living cells 

 soon become distinctly coloured, and before 

 long are injuriously affected. Hence, it is 

 easy to understand why still more poisonous 

 metallic salts may exercise a fatal effect even 

 when much more dilute solutions are em- 

 ployed. Nageli ' found the poisonous char- 

 acter of ordinary distilled water to be due 

 to its containing a trace of copper or other 

 metals, and proved that one part of copper 

 in 1,000,000,000 parts of water sufficed to kill 

 a Spirogyra filament. With a limited amount 

 of the watery solution, the poisonous effect 

 decreases as the number of plants immersed 

 increases, for as the total quantity of the 

 metallic salt is now distributed over a larger 

 number of cells, the lethal limit may no longer be reached in any single cell. 

 If the poisonous effect produced by such dilute solutions were independent of the 

 duration of the period of absorption (which is, however, not the case), and were 

 dependent only upon the final accumulation of a certain quantity of the poisonous 

 substance, then by using sufficiently large quantities of fluid, any solution of 

 a poisonous substance, however much diluted, must ultimately prove fatal. 



The cleansing and purifying power of earth, &c. is the natural consequence of 

 its absorptive properties, properties which play a very important part in Nature. 

 Similarly, it is owing to the absorbent powers of glass that bottles or flasks which 

 have once contained solutions of copper salts or aniline dyes will, even after 

 repeated washing with water, gradually convert the pure water with which they may 

 be filled into a weak and poisonous solution of the substances in question. Plants 



FJG. 9. 



1 Nageli, Oligodynamische Erscheinungen in lebenden Zelleu, 1893 v l>enkschr. d. Schweiz. 

 Naturf.-Ges., Bd. xxxm). On the differences in the manner of death, according to whether the 

 poisonous action is rapid or slow, see Israel, Archiv f. Path. Anat., 1897, Bd. CXLVII, p. 293. 



