Herroun considers that the value of mercuric chloride as an antiseptic is much over-rated, as he has cultivated ordinary septic bacteria in albuminous filtrates, containing 1 in 2000. It is precipitated by albumins if used of greater strength, and is readily converted by the sulphur of albuminous bodies into mercuric sulphide,-a compound which has practically no antiseptic properties.

Sternberg has also made an elaborate series of experiments with regard to the action of germicides. In this case cultivations of well-known pathogenic organisms in liquid media were employed. The supposed germicide was added to the liquid cultivation, and after two hours a fresh flask of sterilised culture was inoculated from the disinfected cultivation, and placed in the incubator. In twenty-four to forty-eight hours, if the chemical were not efficient, there was evidence of a growth of bacteria. Blyth has investigated the disinfection of cultivations of Bacterium termo, of sewage, and typhoid excreta, and, in conjunction with Klein, the effect of well-known disinfectant materials on anthrax spores. Miquel, Laws, and others have also contributed to our knowledge of the effect of antiseptics and disinfectants upon micro-organisms. In spite of all that has been done, there is room for many workers; a great deal of ground must be gone over again to rectify discrepancies, examine conflicting results, and thus determine what

observations may be relied upon for practical application.

Hot Air and Steam.-Koch, in conjunction with Wolfhügel, also tested the value of hot air. A similar plan was adopted as in disinfection with chemicals. Bacteria and spores were subjected for a certain time to a known temperature in a hotair chamber, and then were transferred to nourishing soil, or animals were inoculated.

Paper parcels, blankets, bags, and pillows, containing samples of micro-organisms wrapped up inside, were also placed in the hot-air chamber, to test the power of penetration of heat.

The conclusions from such experiments were as follows:

Sporeless micro-organisms at a little over 100° C. are destroyed in one hour and a half.

Spores of bacilli require three hours at 140° C. If enclosed in pillows and blankets, exposure from three to four hours to 140° C. is necessary. Spores of fungi require one and a half hours at 110° C. to 115° C.

Further experiments showed that at the temperature necessary for the destruction of spores of bacilli almost all fabrics are more or less injured.

Koch, in conjunction with Gaffky and Löffler, also investigated the effect of steam under pressure and at the atmospheric pressure.

Rolls of flannel with anthrax spores or earth spores, and a thermometer wrapped up inside, were

subjected to steam, and the results compared with the effect obtained with hot air.

Thus in hot air four hours' exposure to a temperature of 130° C. to 140° C. brought the temperature inside the roll to 85° C., and the spores were not injured; on the

other hand, exposure to steam under pressure at 120° C. for one and a half hours, raised the internal temperature to 117° C. and killed the spores.

By such experiments the superior penetrative power of steam-heat was established.

To test steam-heat at the atmospheric pressure, water was boiled in a glass flask with its neck prolonged by means of a glass tube, the temperature in which was found to be uniform throughout. Anthrax and earth spores placed in the tube were found to be unable to withstand steam at 100° C. even for a few minutes. It was, therefore, concluded that disinfection by steam at atmospheric pressure was superior to hot air from its greater efficiency, and to steam under pressure from the simplicity of the necessary apparatus.

Parsons and Klein made some experiments which were more in favour of dry heat than the above. These observers state that anthrax bacilli are destroyed by an exposure of five minutes to from 100° C. to 103° C., and that anthrax spores are destroyed in four hours at 104° C., or in one hour at 118° C. Guinea-pigs inoculated with tuberculous pus which had been exposed for five minutes to 104° C.,

remained unaffected. They concluded that as none of the infectious diseases, for which disinfecting measures are in practice commonly applied, are known to depend upon the presence of bacilli in a spore-bearing condition, their contagia are not likely to retain their activity after being heated for an hour to 105° C. (220° F.).

In experiments with steam, the results were in accordance with those already given, and complete penetration of an object by steam-heat for more than five minutes was deemed sufficient. They also arrived at the same result as in Koch's experiments, that steam-chambers are preferable to those in which dry heat is employed, though it must be borne in mind that some articles, such as leather, are injured by exposure to steam.

CHAPTER X.

IMMUNITY.

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THE condition of being insusceptible to infective disease may be either natural or acquired. In the description of the pathogenic organisms several examples of natural immunity will be encountered. The bacillus of septicemia, so fatal to house mice, has been shown to have no effect upon field mice. The bacillus of anthrax is innocuous to cats, white rats, and, it is commonly asserted, to adult dogs and asses. The bacterium of rabbit septicemia is equally inert in dogs, rats, and guinea-pigs. The immunity may be as in these cases complete, or only partial. Ordinary sheep are very easily affected with anthrax, but Algerian sheep only succumb to large doses of the virus. Natural immunity may not only be characteristic of certain species, but it may occur in certain individuals of a susceptible species. The same occurs in man, for certain individuals, though equally exposed during an epidemic of small-pox, may escape where others readily fall victims to the disease.