810 THE POPULAR SCIENCE MONTHLY. 



etc., take a particle of virus, known or supposed to contain patho- 

 genic microbes, from a person suffering from an infectious disease, 

 insert it by a delicate glass pipette through, the sterilized cotton 

 plug of a test-tube containing some of the prepared culture mate- 

 rial, and deposit it there. Then place the tube in an incubator, 

 warmed to the required degree, and let it remain for the number 

 of hours suited to the peculiar requirement of its germ contents. 

 By this means a vigorous progeny of one kind of microbe is 

 obtained, while the tendency is to eliminate other kinds whose 

 requirements are different. 



But to further insure the. exclusion of the ubiquitous horde, 

 take out carefully a little colony of the vigorous microbes of the 

 first culture through the cotton covering and place it in a new 

 culture-tube with the same precautions as before, and so on, until, 

 through high feeding of our test microbes and the adverse treat- 

 ment of the others, we have, by microscopic tests, the thorough- 

 bred, vigorous, and, may be, deadly microbe, which may be seen 

 and every characteristic noted as to size, form, coloring, manner 

 and time of development, all of which enable the observer to fix 

 its classification. But the crucial test as to the relationship of a 

 certain species of microbe to a particular disease is made as fol- 

 lows : Take, as above, the microbes from an individual suffering 

 from a well-known infectious disease, cultivate them to complete 

 isolation and perfection, and introduce them by inoculation into 

 the blood or tissues of a healthy person. Here they must undergo 

 a period of development or incubation, requiring just the number 

 of days and hours as in the culture-tube. This fully developed 

 disease must be strictly the same as that which furnished the test 

 germs. 



In making these experiments with the virus of dangerous 

 diseases the human subject can not, of course, be deliberately 

 employed ; but casual inoculations and infections furnish oppor- 

 tunities for exact observation. A few enthusiastic pathologists 

 and would-be martyrs have submitted to inoculations which have 

 proved of scientific value. The inferior animals furnish much 

 valuable material in this line, although they are entirely exempt 

 from many diseases belonging to man ; while in the human sub- 

 ject there seems a greater general susceptibility to microbic in- 

 fection. 



Founded upon the knowledge of the natural history of the 

 pathogenic microbes has come the only scientific and satisfactory 

 classification of the infectious diseases. It may be stated, as a 

 rule, that the virus of infectious diseases originates either in the 

 bodies of diseased living beings or in decomposing organic mat- 

 ter. When the germs of the virus mature in the living being, 

 ready for reproduction in another person, they produce the acute 



