I02 NATURAL SCIENCE. Feb.. 



microbes was rendered more vigorous, while excessive pressure- 

 killed them altogether ; but with great care he found a certain zone 

 of pressure under which the bacillus of splenic fever was modified 

 in such a way that sheep inoculated with it were protected against 

 the disease. 



M. A. d'Arsonval,2 again, claims that cultures may be attenuated, 

 as well as sterilised, by the use of carbonic acid gas under high 

 pressures. 



MM. Hericourt and Richet^ mention three ways in which cul- 

 tures may be attenuated so as to be suitable for protective inocu- 

 lation, viz. : — 



1. Making the culture in a less suitable medium, {e.g., beef-tea 



not peptonised). 



2. Keeping the cultures till they have passed the period of their 



most vigorous growth. 



3. Cultivating above or below the most favourable temperature 



for growth. 



The general principle of attenuation seems to be, to make the 

 cultures under slightly unfavourable conditions, by which the 

 microbes are rendered less vigorous. 



It is not, however, so much with these interesting experiments 

 that I am concerned at present as with the explanation offered by 

 Pasteur and others of the way in which these attenuated viruses 

 are supposed to confer immunity from disease. 



There are two general views, the first being that the microbe 

 of the " modified cultivation exhausts the soil," that is to say, 

 deprives the blood or tissues of something necessary to the growth 

 of the unmodified form. This is known as the Exhaustion theory. 

 The second view is that some product of growth — some chemical 

 secretion of a toxic character — of the modified virus is inimical to the 

 growth of the original. This is the Antidote theory. 



Bacteria belong to the vegetable kingdom, and both theories 

 have the support of analogy, that is to say, both explanations have 

 been applied to the case of the higher plants. It is well known that 

 after repeated crops of any species of plant on a particular spot, the 

 land seems, as it were, to become tired of it, and the growth is less 

 vigorous. Two explanations are offered : first, that the particular 

 plant has exhausted the soil of something specially needful for its 

 growth — this is the exhaustion theory ; secondly, that the plant renders 

 the soil unsuitable for another of its own species by polluting it with 

 its excretions — this is the antidote theory. 



The former is the explanation most generally received, and is 

 the one which has obtained the support of Professor Tyndall, whose 

 exact words may here be quoted : — 



'* Now contagia are living things, which demand certain elements 

 of life, just as inexorably as trees, or wheat, or barley ; and it is not 



'■^ Comptcs Rendus, vol. cxii., p. 6C7. ^ Comptcs Rcndus, vol. evil., p. 690. 



