bacteria in air over foaming liquids. B. prodigiosus per liter.

Experiment.

In liquid.

In air.

1

............................................

630,000,000

0

0

0

0

2

............................................

680,000,000

0 0

0

0

0

3

............................................

230,000,000

1

0

0

4

............................................

5,000,000,000

0

1

3

0

5

............................................

1,000,000,000

0

0

0

0

6

............................................

2,700,000,000

1

0

0

1

7

............................................

1,800,000,000

0

0

0

8

............................................

4,400,000,000

0

0

0

He found that even under conditions of foaming and bubbling very favorable for ejecting bacteria into the air from liquids containing many millions and even billions of bacteria per liter,* the number so ejected was so very small as to be practically negligible. Most accurate methods of detection were employed, so that there should be no question of detection of any germ released from the liquids swarming with them. Prof. Newman** gives a provisional list of normal sewage bacteria as follows: (1) Coli communis, (2) proteus vulgaris, (3) B. enteriditis sporogenes, (4) liquefying bacteria, B. subtilis and B. mesentericus, (5) non-liquefying bacteria, (6) sarcinae, yeasts and moulds. No pathogenic bacteria are included in this inventory. "Doubtless," he says, "such species (e. g., typhoid) not infrequently find their way into sewage. But they are not normal habitants, and though they struggle for survival, the keenness of the competition among the dense crowds of saprophytes makes existence almost impossible for them. * * * There is no relationship between the microbes contained in sewer air and those contained in sewage. Indeed, there is a marked difference which forms a contrast as striking as it is at first sight unexpected. The organisms isolated from sewer air are those commonly present in the open air. Micrococci and moulds predominate, whereas in sewage bacilli are most numerous. * * * Pathogenic organisms and those nearly allied to them are found in sewage, but absent in sewer air. ** * * Lastly, only when there is splashing in the sewage, or when bubbles are bursting (Frankland) is it possible for sewage to part with its contained bacteria to the air of sewers. * * * The interior of the cavity of the mouth and external respiratory tracts is a moist perimeter from the walls of which no organisms can rise except under molecular disturbance. The position is precisely analogous to the germ-free sewer air as established by Messrs. Laws and Andrewes for the London City Council. The popular idea that infection can be 'given off by the breath' is contrary to the laws of organismal pollution of air. The required conditions are not fulfilled, and such breath infection must be of extremely rare occurrence. The air can only be infective when filled with organisms arising from dried surfaces. The other series of investigations were conducted by Drs. Hewlett and St. Clair Thomson, and dealt with the fate of micro-organisms in inspired air and micro-organisms in the healthy nose. They estimated that from 1,500 to 14,000 bacteria were inspired every hour. Yet, as we have pointed out, expired air contains practically none at all. * * * From the two series of experiments which we have now considered we may gather the following facts :

*See report of Prof. C. E. A. Winslow's experiments for the National Association of Master Plumbers.

•About a quart.

** Demonstrator of Bacteriology in King's College, London, in his work entitled "Bacteria," published by John Murray, London, 1899.

(a) That air may contain great numbers of bacteria which may be readily inspired.

(b) That in health those inspired do not pass beyond the moist surface of the nasal and buccal cavities.

(c) That here there are various influences of a bactericidal nature at work in defense of the individual.

(d) That expired air contains, as a rule, no bacteria whatever.

* * * "It should be noted that the bacilli of diphtheria are capable of lengthened survival outside the body, and are readily disseminated by very feeble air currents."

Miquel found during a six years' investigation of the air of Paris an average of 4,000 bacteria per cubic meter in that of Montsouris Park. Flugge, taking an average of the middle of the city, but only about a tenth as many in 100 bacteria per meter, estimates that "a man during a lifetime of seventy years inspires about .25,000,000 bacteria, the same number contained in a quarter of a liter of fresh milk."

The number of bacteria in the air diminishes in cities rapidly in proportion to the altitude, Miquel finding 750 per cubic meter in the Rue de Rivoli, but only 28 at the summit of the Pantheon. Whereas at the seashore there might average a hundred per meter, the number diminishes as the distance seaward increases, and the maximum distance seaward, according to Dr. Fischer's experiments, to which germs can be transported lies between 70 and 120 miles, beyond which they are almost invariably absent. "Of particular interest in these experiments," says Frankland, "is the very distinct manner in which they show that the microorganisms which are present in sea-water are not communicated to the air, excepting in the closest proximity to the surface, even when the ocean is much disturbed."

All these facts co-operate to show that wet surfaces do not give up their germs to the air under normal conditions.