SELECTIVE AFFINITIES. l^ 



The chemical material, it may be assumed, could act in two ways. 

 It might act as a substance without which the microbe could not 

 live, or it might act either as a poison to the microbe, or as an 

 antidote to or a remover of a poison formed by the organism itself 

 under its new condition. In some animals, then, it will altogether 

 inhibit the pathogenetic activity of the organism ; in others, where it 

 is not so constituted as to counteract the morbific products, or the 

 activity which gives rise to them, the organism will continue to pro- 

 duce them, and either the cell elements of the host or those of the 

 attacking organism will succumb to their action. 



In artificial media it is a frequent experience to find organisms 

 destroyed by the virulence of their own products, before all the food 

 material adapted to their existence has been exhausted ; and the 

 chnical evidence of those infective diseases in which a crisis occurs 

 {e.g., croupous pneumonia) goes far to support the view that a similar 

 process takes place at times within the body. 



14. M. Raulin, in his experiments upon Aspergillus nigrescens, found 

 that although it could grow readily on bread moistened with vinegar, 

 it flourished much more luxuriantly on an artificially prepared medium, 

 in which certain definite inorganic materials are present. — (See 

 Raulin's liquid, § 58, p. no). He found that under ordmary con- 

 ditions 25 grammes of dried aspergillus is regularly obtained from a 

 definite quantity of the liquid before the whole of the elements are 

 exhausted. If the small proportion of potassium be removed, only 

 one gramme of the dried aspergillus is produced, whilst if the zinc be 

 omitted from the liquid (only -g^,^^^ part of the mixture), the crop 

 gathered amounts to 2-5 grammes only. This zinc is found in small 

 quantities in the aspergillus, and is therefore taken up as one of its 

 alimehts. Most minute quantities of nitrate of silver {-x,-^-^,-^^, of 

 corrosive sublimate (-nf.^^nr)) °f bichloride of platinum (^.^nnr), &c., act 

 as poisons upon this same aspergillus, and prevent its growtli. 

 Duclaux, in commenting upon these facts, points out that "as the 

 plant does not contain any green matter, it may be surprising to see 

 that iron is one of its nutritive elements. Indeed, the withdrawal of 

 that metal produces results similar in importance to those produced 

 by the suppression of zinc. The addition of one gramme of iron to 



