102 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,? again, claims that cultures may be attenuated, 
as well as sterilised, by the use of carbonic acid gas under high 
pressures. 
MM. Héricourt and Richet3 mention three ways in which cul- 
tures may be attenuated so as to be suitable for protective inocu- 
lation, vizs-— 
1. Making the culture in a less suitable medium, (¢.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 Iam 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 
2 Comptes Rendus, vol. cxii., p. 667. 5 Comptes Rendus, vol. cvii., p. 690. 
