THE TOXINS 165 



cipitated sesquioxide of iron absorbs arsenious acid (and 

 probably also phosphoric acid) till there remains in the fluid 

 less than one-thousand-millionth part (A. Gautier). A skein 

 of white silk dipped in a solution of eosin so dilute that the 

 eye can perceive no colour, i.e., to about one in a million, is 

 dyed pink in the course of a few hours. Silk can take up 

 I "3 per cent, of picric acid from a solution containing only 

 o'oo6 per cent. The absorptive power increases the greater 

 the dilution. Passing to living cells, we find that a culture of 

 Aspergillus niger can take up from a solution measuring 

 250 c.c. and containing one-half milligram of zinc, i.e., 

 1/500,000, practically the whole of this metal (Javillier) ; the 

 proportion of zinc increases to about i in 10,000 in the 

 aspergillus cells and falls below i in 10,000,000 in the fluid 

 which remains. Certain plants can absorb copper from 

 solutions containing only i in 100,000,000. Certain marine 

 plants, such as the ordinary sea-wrack, fix abundantly the 

 iodine and silver which exist in traces only in sea-water. 



The whole industry of dyeing is founded on similar fixations : 

 fabrics are sensitive to and fix selectively the colouring 

 materials. Picric acid, which stains the skin, does not dye 

 cotton. The microscopical preparations of histologists and 

 biologists depend on this principle of the selective fixation of 

 the stains by different anatomical elements : magenta is fixed 

 by the nucleus, picric acid by the protoplasm, indigo-carmine 

 by the connective-tissue fibres. 



Ehrlich has pointed out that in jaundice the kidneys and the 

 liver become charged with bile-pigments, whereas the brain 

 remains free. When certain derivatives of paraphenylene- 

 diamine are administered to mice, the central part of the 

 diaphragm is found stained brown much more intensely than 

 the periphery, and the muscles of the eyes, of the larynx, 

 and of the tongue are much more deeply stained than other 

 muscles : this may be because these muscles are in continual 

 activity, receive more blood, and are the seat of more intense 

 oxidations. Methylene blue in the living animal is fixed by 

 the sensory fibres, by the nerve-endings for taste and smell. 



