heifer, by inoculation with lymph which has been "humanized" by long transmission through a succession of human beings. For, as is proved by Dr. Martin's experiments on this retro-vaccination, such lymph has been so altered by "humanization" that the germs it contains do not properly reproduce themselves in the system of the calf, thus showing that it no longer possesses the attributes of true vaccinia. And, although the liability to contamination from human disease may be thus greatly diminished, it can not be certainly said to have been destroyed.
We now come to the bearing of Pasteur's researches on the question of the fundamental identity of small-pox and cow-pox, originally mooted by Jenner. Attempts at its solution were made, early in the present century, by the inoculation of bovine animals with small-pox virus; and it was asserted that in this way true vaccinia had been artificially produced. But the evidence in support of this assertion did not command general assent; and it was not until Dr. Thiele, of Kazan, published, about forty years ago, an account of his experiments, that the doctrine obtained any considerable amount of acceptance. According to the citations given by Mr. Simon, in his valuable "Report on Small-pox and Vaccination," issued under Government authority, in 1857, Dr. Thiele not only repeatedly succeeded in producing a genuine vaccinia by inoculating bovine animals with small-pox virus, but himself used this artificial vaccine largely and successfully in human vaccination, and propagated it extensively by the instrumentality of other vaccinators, its protective power being found to be fully equal to that of the natural vaccinia. But, further, Dr. Thiele asserted that he could produce this artificial vaccine without the use of the cow at all, by diluting the small-pox virus with warm milk, or, as we should now term it, "cultivating" its living germs in that fluid. I can scarcely help thinking that the great improbability as it then seemed—of such a conversion has thrown a discredit upon the whole of Dr. Thiele's statements, which has caused them to be ignored by most subsequent workers on this subject. But, should that part of his results be ever confirmed, he must be accorded the credit of having anticipated in a most remarkable way one of the most important of Pasteur's methods, though, it is pretty certain, without knowing, or even guessing, their true rationale; for it must have been not by dilution of the virus (like that of a chemically acting fluid), but by a modification in the character of the disease-germs resulting from their development in milk, that this part of Thiele's results (supposing them to be genuine) was produced.
Simultaneously with those of Dr. Thiele, a set of experiments of the same kind was being carried on in our own country by Mr. Ceely, of Aylesbury; the results of which, however, were not equally satisfactory. He did, it is true, produce an eruption in cows inoculated with small-pox virus, which was transmissible by inoculation to the