32 THE POPULAR SCIENCE MONTHLY. 



temperature does in these cases chemical products, special to cer- 

 tain animals, can also bring about in others. This is well proved 

 by the influence of various culture media on the growth of micro- 

 organisms. The presence of chemical compounds of well-known 

 nature, even in very small quantities, has been shown by a great 

 many observers to influence much the mode of growth of bacteria. 

 It has even been proved that bouillon obtained from the muscular 

 tissue of various animals, notwithstanding the absence of any 

 very definite active chemical compound causing marked differ- 

 ence, gave cultivation media more or less suitable for the growth 

 of certain organisms. I will only mention out of a large number 

 of other observations the very recent experiments of Hippolyte 

 Martin on the Bacillus tuberculosis. This observer found that 

 animals can be classified roughly in .the following way, according 

 to the ease with which the bacillus grows in bouillon made with 

 their tissues : herring, oyster, mussel, monkey, horse, calf, rabbit, 

 birds, dog, cat, rat. It would be difficult in the present state of 

 our knowledge to ascribe these differences to the presence of any 

 definite compound, yet it can not be doubted that they are due to 

 certain physico-chemical properties. We have thus distinct evi- 

 dence of marked differences between animals of different classes, 

 orders, and species ; and, if we admit the truth of the doctrine of 

 evolution, we must admit that such differences are in great part 

 the result of the influence of external circumstances. We might 

 infer from this that differences between animals of the same spe- 

 cies, but of different races, families, sex, or age, are likewise the 

 result of similar influences ; we have, however, better evidences 

 than these in support of the view that either extreme liability or 

 immunity to disease may be acquired. Indeed, I shall be able to 

 show you that it is the gradual development of that knowledge 

 which has prepared man for the reception of Pasteur's discoveries, 

 and of their recent momentous extension by Koch. 



Refractory State resulting from a First Attach of Certain 

 Fevers. It was very early recognized that after a first attack of 

 many infectious fevers, such as small-pox, measles, typhus fever, 

 etc., a second attack seldom occurred. This fact seems to have 

 been observed more specially in connection with small-pox, or at 

 any rate to have led to practical application first in connection 

 with that disease. We learn that the inducement of a first attack 

 of small-pox was an antique practice in Africa, Persia, and China, 

 and that the method of inoculation was brought from there to 

 Constantinople in 1673, and from that town to England by Lady 

 Mary Wortley Montagu. The idea was evidently to produce a 

 mild attack of the disease in individuals placed under circum- 

 stances most favorable to recovery in order to induce immunity. 

 The practice, although open to serious objections, must have had 



