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that the means of disinfection resorted to were efficient. Negative evidence should be received with great caution. In the first place, the question as to whether susceptible individuals have been fairly exposed in the disinfected room must be considered. Then it must be remembered that susceptible persons do not always contract a disease, even when they are exposed in a locality known to be infected. A further difficulty in estimating the value of evidence obtained in practice arises from the fact that in connection with the special means of disinfection resorted to, such as fumigation, hanging up cloths saturated with a disinfecting solution, etc., it is customary to resort to additional precautionary measures, such as washing surfaces with soap and hot water, whitewashing plastered walls, and free ventilation. It is apparent that under these circumstances it would be unsafe to accept the fact that no other cases occurred in a room treated in this way as evidence that the particular disinfectant used is efficient for the destruction of the infectious agent of the disease in question. The fond mother who attaches a charm to her child's neck to protect it from evil also takes the precaution of guarding it from contact with other children who are sick with any infectious disease. If her child fortunately grows to manhood or womanhood without having suffered an attack of scarlet fever or diphtheria, she may imagine that her charm has protected it; but the evidence upon which her faith is founded is not of a nature to convince those who are familiar with scientific methods of demonstration. “Well educated” persons are often ready to testify in favour of methods of disinfection or of treatment upon evidence which, from a scientific point of view, has no more value than that which the fond mother in question has to offer in favour of the little bag containing camphor or assafotida or some other charm of equal value which she has attached to her child's neck to keep it from catching scarlet fever or diphtheria at school. On a par with these charms, so far as disinfection is concerned, we may place the saucer of chloride of lime, which it was formerly the fashion to place under the bed of a patient sick with an infectious disease, the rag saturated with carbolic acid or chloride of zinc, suspended in the sick-room, and even the fumigations with burning sulphur, as sometimes practised by those who are unfamiliar with the evidence as to the exact value of this agent and the conditions necessary to insure successful disinfection with it.
Chloride of lime, sulphurous-acid gas, and carbolic acid are among our most useful disinfecting agents; but disease germs cannot be charmed away by them any more than by a little bag of camphor.
Having pointed out the fact that negative evidence, in a restricted field of observation, must be accepted with great caution in estimating the value of disinfectants, we hasten to say that the combined experience of sanitarians, derived from practical efforts to restrict the extension of infectious diseases, is of the greatest value, and that this experience is, to a great extent, in accord with the results of exact experiments made in the laboratory.
(6) Inoculation experiments upon susceptible animals, made directly with infectious material which has been subjected to the action of a disinfectant, have been made by numerous observers. The proof of disinfection in this case is failure to produce the characteristic symptoms which result from inoculation with similar material not disinfected. Thus, Davaine found that the blood of an animal just dead from the disease known by English writers as anthrax or splenic fever, inoculated into a healthy rabbit or guinea-pig, in the smallest quantity, infallibly produces death within two or three days; and the blood of these animals will again infect and cause the death of others, and so on indefinitely. This anthrax blood therefore is infectious material, which can be utilised for experiments relating to the comparative value of disinfectants. Davaine made many such experiments, not only with the blood of anthrax, but also with that of a fatal form of septicæmia in rabbits, which is known by his name. Other investigators have followed up these experiments upon infectious material of the same kind, and also upon material from other sources—the infectious material of glanders, of tuberculosis, of symptomatic anthrax, of fowl cholera, of swine plague, etc.
It has been proved that the infectious agent in all of the diseases mentioned is a living germ, and that disinfection consists in destroying the vitality of this germ. But in experiments made with blood or other material obtained directly from diseased animals, the results would be just as definite and satisfactory if we were still ignorant as to the exact nature of the infecting agent. The test shows the destruction of infecting power without any reference to the cause of the special virulence, which is demonstrated to be neutralised by certain chemical agents in a given amount. All of the experiments made with the above-mentioned kinds of virus have been made upon the lower animals; but there is one kind of material which it is justifiable to use upon man himself, and with which numerous experiments of a very satisfactory character have been made. This material is vaccine virus. Fresh vaccine, when inoculated into the arm of an unvaccinated person, gives rise to a very characteristic result — the vaccine vesicle. The inference seems justified that any agent which will neutralise the specific infecting power of this material will also neutralise the smallpox virus. In these experiments the more careful investigators have taken the precaution of vaccinating the same person with disinfected and non-disinfected virus from the same source. A successful vaccination with the non-disinfected virus shows that the individual is susceptible and the material good; failure to produce any result is evidence that the potency of the disinfected virus has been destroyed by the chemical agent to which it was exposed.
(c) As already stated, it has been demonstrated that the infectious diseases of the lower animals, which have furnished the material for experiments upon disinfectants by the method of inoculation, are “ germ diseases,” and that the infectious agent is in each case a living micro-organism, belonging to the class known under the general name of Bacteria. The bacteria are vegetable organisms, which, by reason of their minute size and simple organisation must be placed at the very foot of the scale of living things; but they make up in number and in rapidity of development for their minute size.
Many of these disease germs are now known to us, not only by microscopic examination of the blood and tissues of infected animals, but also by “culture