STERILISATION. 29 



As stated above, it has been proved that very minute 

 quantities of poisons may have such an influence on micro- 

 organisms that they actually stimulate them to more rapid 

 growth, often one-sided ; it may be the development of the 

 vegetative organs at the expense of the organs of propagation, 

 or again by bringing about an increase of the fermentative 

 activity. In a few cases that have been closely examined it 

 has been proved that the minute doses which can produce 

 such an action have fairly well-defined limits ; the least excess 

 brings about the opposite action a weakening of the organisms 

 in question. Thus a minute dose of a copper salt assists the 

 development of the mould, Aspergillus niger, to a very great 

 extent. In the same way Biernacki found that the addition 

 of copper sulphate in the proportion of 1 : 600,000, added to 

 the nutritive value of the liquid, and increased the activity 

 of the ceUs. In larger quantities copper salts exercise a 

 destructive action on yeast ; care must, therefore, be taken 

 that when pure cultures of yeast are introduced into copper 

 vats, these should be carefully tinned. Hayduck (1881) 

 found that small quantities of lactic acid (about 0-5 per cent.) 

 favour the propagation of yeast, and that anything up to 

 1 per cent, of lactic acid, under the usual technical conditions, 

 is favourable to the production of alcohol. Heinzelmann 

 proved in 1882 that salicylic acid in the proportion of 0-1 

 gramme per litre reacts favourably on yeast cells, and H. 

 Schulze (1888), that minute traces of poisons, such as mer- 

 curic chloride, iodine, chromic acid, and lactic acid, have the 

 same action (e.g., mercuric chloride in a dilution of 1 : 500,000). 

 Hirschfeld found that by the addition of 0-01 to 0-02 per cent, 

 of hydrochloric acid the acetic fermentation is very consider- 

 ably quickened. Richet proved a similar relation holds good 

 with the lactic acid bacteria, while the addition of 0-5 mgm. 

 of mercuric chloride, or of copper sulphate, per litre, intensifies 

 their fermentative activity. In the same way Effront found 

 that minute quantities of sulphuric acid and of fluorides have 

 a stimulating action in nutritive liquids, both on the rate of 

 propagation and the fermentative capacity of yeasts, but 

 that this varies with the yeast species. 



The fungi have a curious power of accommodating them- 



