164 RECENT RESEARCHES ON THE BACTERIA. 



change of climate, sea-voyages, the air of pine-woods, inhalation 

 of Carbolic Acid, etc. If the Bacilli were the sole cause of 

 consumption, as all must be exposed to the risk of contagion, it 

 would seem that no one could possibly escape ; but, as it is only the 

 minority who succumb, it is probable that there must be some 

 prior condition which favours its development, — a state of health 

 or constitution which is a main factor in the production of the 

 disease, and which is as necessary to the growth of the Bacilli as 

 is the manuring and preparation of a garden for the crop which is 

 to grow in it. 



Not only air but also water may convey poison germs into our 

 system; and the reason why water contaminated with decomposing 

 vegetable matter, or sewage, is so fruitful a source of disease, is, 

 that such water supplies suitable nutriment to all the bacterial 

 germs, which grow and propagate therein, and not only propagate 

 but develop more intensely poisonous characters in such contami- 

 nated water ; and we thus see how it is that boihng renders such 

 water innocuous by destroying the contained Bacteria. Professor 

 Mathieu Williams has thus accounted for the density of the popu- 

 lation of China, from the fact that, in spite of the general insanitary 

 condition of the country, the small amount of cleanliness, and the 

 crowded condition of the people, the usual drink consists of 

 boiled water, flavoured with an infusion of tea-leaves, which may 

 thus be a great protection against the spread of zymotic disease. 



The study of Bacterial disease has also helped us to understand 

 the connection between vaccination and small-pox. Jenner 

 suspected that cow-pox might be small-pox modified by passing 

 through the living cow ; since his time, however, the prevailing 

 opinion has been that they are two distinct diseases ; but recent 

 observations seem rather to prove that they are modifications of 

 the same poison : that is to say, that Cow-pox (or the disease, 

 produced by vaccination) is actually human small-pox modified by 

 its passage through an animal of the bovine species. The discov- 

 ery that the poison of Anthrax may be mitigated by passing it 

 through the system of a living rodent, and that on retro-vaccination 

 with this lymph in the body of a living ruminant, it forms a pro- 

 tection against a fresh attack, appears to support this theory. 

 While the facts that cow-pox, as it occurs naturally, is a disease 



