rainfall and during the winter the number of germs in the air is considerably diminished. In the still air of rooms comparatively few germs are found, even in densely occupied quarters-as, for instance, in hospital-wards. As soon, however, as dust is set in motion, the number of microorganisms in the air increases enormously-to as many as 16,000 in the cubic meter. The majority of the bacteria, however, by reason of their weight, are soon again, after from half an hour to an hour, deposited upon the floor and the walls with the dust, and the air will then contain almost only the light mold-spores.

Sources of Aerial Germs (Air-infection). The presence of spores of molds in the air in such large number is explained by the fact that generally the fruit-bearers of the fungous deposits project upward from the mycelium, and the spores can, therefore, be readily conveyed by currents of air.

Bacteria frequently find their way into the air with the small fibers separated from clothing, handkerchiefs, linen,

etc.

With regard to the possibility of air-infection, recent investigations by Flügge and his pupils have yielded a wealth of facts that are well adapted to modify the views hitherto held. The earlier supposition that no germs enter the air from liquids or from moist surfaces is, according to Flügge, valid only in so far as the surface of the fluid remains unmoved and intact. A moderate wind (a velocity of four meters in a second) suffices, however, to set free germcontaining small drops from water-surfaces, moist articles, and the like. The conditions favoring the conversion of fluids into small drops in the air are often present, as in the open air, in a high degree in the neighborhood of the surging sea, of water-mills, etc., in lesser degree with every current of air that sets the leaves of trees in motion. In closed apartments, according to Flügge, small drops that enter the air are set free more frequently than is generally believed in sprinkling, in washing, in manipulating wet linen, and—upon which especial weight must be placed— in speaking, sneezing, and coughing. These droplets, whether they contain germs or not, are, as Flügge further demonstrated, carried away by the slightest currents of air for considerable distances, and not only in a horizontal, but also in a vertical direction. In order to demonstrate the

dissemination of such droplets in speaking, coughing, and sneezing, Flügge had the person under observation introduce a suspension of prodigiosus into the mouth. All the observations yielded positive results: The plates exposed at a distance of several meters became covered by characteristic colonies, and they remained sterile only after quiet speaking in a low tone.

With regard to the separation of desiccated germs, it may be said that this may likewise occur frequently. Flügge found that the movement of the finest dust already begins with a current of air having a velocity of one meter. In the open air the most varied mechanical influences (wagonwheels, pedestrians) cause the separation of the smallest particles, which are then readily borne by the wind. rooms particles of dust and fibers are set free by the vibration of the floor, by manipulation of utensils, furniture, clothing, etc. The smallest of these floating particles containing germs are, however, as Flügge has shown, moved in a horizontal direction by minimal currents of air of 0.2 mm., and in a vertical direction by currents of from 0.3 to 0.4 mm. per second. In the course of his experiments Flügge succeeded with the prodigiosus in demonstrating the distribution of dust-germs in all possible parts of a closed room. Germ-containing dust is not completely removed from rough surfaces even by the strongest currents of air. Airing of infected clothing, as frequently practised, does not, therefore, entirely attain the object desired.

Upon the basis of the results of these observations Flügge expresses the view that in the case of all infectious diseases air-infection may take place through the smallest droplets separated from fluid sources of infection. In cholera and typhoid fever this will occur but rarely-for instance, in the sprinkling of polluted water, in the washing of linen, and the like. The principal rôle in these affections is played by contact-infection, the dissemination through drinking-water, articles of food, etc. In the infectious diseases of the nose, the pharynx, and the respiratory organs, however, the possibility of transmission through small drops must be given greater consideration than it has received in the past. In diphtheria, influenza, whoopingcough, pneumonia, and pulmonary tuberculosis, droplets. containing germs are sent into the air in coughing,

sneezing, and loud talking; they are set in motion by minimal currents of air, and in a quiet room can be demonstrated floating in the air for as long as five hours.

In cases of air-infection through dry particles of dust, only the finest particles, capable of being carried by any movement of air, demand consideration. Only such diseases are transmitted naturally in this way whose exciting agents are still capable of surviving in the dry state. For the acute exanthemata, which have always been considered as diseases due to volatile contagia, such a possibility must be accepted.

Pulmonary tuberculosis has hitherto been generally accepted as disseminated through inhalation of ejected dried and pulverized sputum containing tubercle-bacilli ; Flügge opposes this view. Attempts to infect animals through inhalation of dried tuberculous sputum have never quite succeeded; and, further, the tubercle-bacilli do not appear at all capable of being carried by the finest particles of dust. Flügge expresses a warning against going to the other extreme of attaching too great importance to the danger of infection through drops of spray or even of considering it the only source of infection. Infection with tuberculosis is dependent upon a large number of factors: upon the environment of the patient, upon the presence of tubercle-bacilli in the saliva, which forms fine droplets much more readily than the viscid sputum proper, and upon other conditions.

WATER.

Method of Examination.-The water to be examined is collected in sterilized Erlenmeyer flasks, and is investigated as speedily as possible. If the examination is deferred for only a few hours, the indifferent saprophytic bacteria that reside in water will have undergone multiplication, and the estimation of the number of water-germs will not yield reliable results. If the specimens of water to be examined are sent from a distance, the water must be forwarded in sterilized flasks, provided with glass stoppers, and packed in ice. In obtaining water for examination care should be taken that the material to be examined has not stagnated in the conduit from the well or spring or other source of supply. For this reason a certain amount of water is first

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permitted to escape. Of the specimen obtained, 1, 1⁄2, and 4 cu. cm. are removed by means of sterilized pipets, and introduced into liquefied gelatin, and plates are made. Greatly polluted water should be diluted with sterilized water from another source, and no more than 1, 10, and Tooo cu. cm. should be employed. Before introducing the pipets the specimen is vigorously agitated, as the bacteria, by reason of their weight, quickly settle to the bottom. In the process of water-examination Koch's old method of making plates is still frequently followed, because these permit better than the Petri dishes the distribution of the gelatin in as uniform a layer as possible. When the plates have developed, the colonies are examined and counted, if present in large number by means of a special enumerator (a glass plate with etched squares). An estimate is made of the number of germs to the cubic centimeter of water, and the number as well as the identity of the varieties present is determined.

The Number of Bacteria Present in Water.-Springwater, as well as subsoil-water, is free from germs at the point where it escapes from the earth. It has been mentioned that the earth at the level of the subsoil-water no longer contains bacteria. According to the most trustworthy investigations, pure tap-water and spring-water contain, on the average, from 2 to 50 bacteria per cubic centimeter; pure pump-water, from 100 or 200 to 500 germs; unfiltered water from streams kept unpolluted, from 6000 to 20,000; filtered river-water, from 50 to 200; polluted wells, as many as 100,000 germs; and river tap-water, when the filtering apparatus is out of order, the same number. Drain-water and greatly contaminated streams contain from 2,000,000 to 40,000,000 germs to the cubic centimeter (Flügge). In the summer and after a heavy rainfall the number of bacteria in water is increased.

The Bacteria of Water.-The microorganisms living in water are mainly bacilli. A large number of them liquefy gelatin; others generate offensive gases, and still others beautiful pigments. Of great interest are the so-called typhoid-like water-bacteria (p. 402) and the water-vibrios, which in a number of points resemble the comma-bacilli of Asiatic cholera (p. 404).

Among pathogenic bacteria typhoid-bacilli and choleravibrios have been repeatedly found in water. The method

for cultivating these bacteria from water has been considered in the discussion of typhoid fever (p. 178) and of cholera (p. 195). At this point it may be briefly repeated that by the addition of one per cent. peptone and 1⁄2 per cent. sodium chlorid, the water to be examined itself constitutes a nutrient medium, so that in this way large quantities of the fluid can be employed, whereas in earlier investigations a fraction of a drop had to suffice, and, as a result, the pathogenic germs were readily overlooked.

The saprophytic water-bacteria multiply in water to an unlimited degree. For the pathogenic bacteria, however, while the possibility of preservation often exists, but rarely is opportunity for proliferation in water afforded. end there are necessary a favorable external temperature (summer) and, further, solid particles of vegetable or animal origin, to which the bacteria adhere, which serve as nutrient medium, and which, at the same time, afford the bacteria protection from the competition of saprophytes. The exciting agents of typhoid fever and of cholera appear to survive in ordinary water for days, and even for weeks. Some observers even believe that cholera-vibrios may at first undergo multiplication in water. As a rule, however, the pathogenic germs are suppressed sooner or later in water in consequence of overgrowth by the saprophytes.

Self-purification of Water.-The microorganisms present in water are derived from the surface of the earth, the air, from waste water and the sewage of cities, emptying into the water-courses, from cesspools communicating with imperfectly constructed subsoil-wells, etc. The pollution of streams by cities is a most prominent factor; the Seine at Ivry, for instance, contains 32,500 germs to the cubic centimeter; below Paris, in Asnières, the number is 12,800,000. Fortunately, if renewed contamination does not take place, streams purify themselves (Pettenkofer). The microorganisms settle and are carried to the bottom, partly with the constituents suspended in the water and with the insoluble earthy combinations that form from calcium and magnesium bicarbonates after escape of the carbon dioxid. Light also in a high degree exerts an injurious influence upon the microorganisms present in water down to a depth of about two meters. The organic substances contained in water are gradually consumed by bacteria and algæ.