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surface soil-we know far too little about them to say much as to the comparison the water is falling more and more out of the dangerous stages.

At still deeper levels, even these anaerobic forms are left behind, and the thoroughly filtered liquid may now subside into a closed subterranean basin, where it may remain pure for any length of time,* so far as living organisms are concerned, provided no fissures or direct prolongations of surface waters allow of contamination from above.†

It is obvious from the foregoing that the two great sources of contamination of our water-supplies are the air and the surface waters.‡ *The remarkably impure deep well water examined by Rohn and Wichmann ('Mitth. d. Oesterr. Versuchstat. f. Brauerei u. Malzerei,' H. 2) must surely have been connected with surface waters!

A recent examination (1891) made by one of us of the water from deep wells in the chalk of the Kent Waterworks Company showed the number of microorganisms revealed by the gelatine test to vary from 4 to 76 and to average 32 in 1 cubic centimeter. In all cases the water was taken directly from the pumps and before it had undergone any storage.

For collected results of the bacteriological examination of spring and well waters, see especially Hueppe ("Die hygienische Beurtheilung d. Trinkwassers vom biologischen Standpunkte," Schilling's 'Journ. f. Gasbeleuchtung und WasserverForgung,' 1887), also Tiemann-Gärtner (Untersuchung. d. Wassers,' Braunschweig, 1889, p. 498).

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‡ It is hardly necessary now to insist on the importance of the air, and its dust, in this connexion. Reference may be made to the following in confirmation :Angus Smith (Air and Rain,' 1872), Tyndall (Floating Matter of the Air,' 1881); Percy F. Frankland (“The Distribution of Micro-organisms in Air," Roy. Soc. Proc.,' 1886, No. 245, p. 509; "Some of the Conditions affecting the Distribution of Micro-organisms in the Atmosphere," 'Soc. of Arts Journ.,' 1887, vol. 35, p. 485; "A new Method for the Quantitative Estimation of Micro-organisms in the Atmosphere," Phil. Trans.,' 1887, B, p. 113); Grace C. and Percy F. Frankland ("Studies on some new Micro-organisms obtained from Air," Phil. Trans.,' 1887, B, p. 257); Percy F. Frankland and T. G. Hart ("Further Experiments on the Distribution of Micro-organisms in Air," Roy. Soc. Proc.,' vol. 42, 1886, p. 267); Miquel ('Annuaire de l'Observatoire de Montsouris,' 1877 to 1891, and 'Manuel Pratique d'Analyse Bactériologique des Eaux,' 1891). Also Aitken in 'Proc. Roy. Soc. Edinb.,' vol. 16, 1886, p. 139; 'Trans. Roy. Soc. Edinb.,' vol. 15, February 6, 1888; and as regards contamination by surface waters see Koch (Rede zur Stiftungsfeier der militärärztlichen Bildungsanstalten,' 1888, p. 25), Plagge and Proskauer (Zeitschr. f. Hyg.,' vol. 2, p. 479), Soyka (Deutsche Vierteljahrschr. f. öffentl. Gesundheitspflege,' 1888, p. 638), Wolffhügel (Arb. a.d. Kaiserl. Gesundh.-amte,' 1886, p. 546), and Fraenkel (Zeitschr. f. Hyg.,' 1889, p. 23).

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As regards filtration, Percy F. Frankland (" On the Removal of Micro-organisms from Water," Roy. Soc. Proc.,' 1885, "New Aspects of Filtration and other Methods of Water Purification: The Gelatine Process of Water Examination," 'Journ. Soc. Chem. Ind.,' 1885; "Water Purification: its Biological and Chemical Basis," Proc. Institut. of Civil Engineers,' vol. 85, 1885-86; "Filtration of Water for Town Supply," "Trans. of the Sanitary Institute of Great Britain,' vol. 8, 1886;

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It is also clear that pathogenic forms find their access to such waters by the same routes as saprophytic and harmless ones, a point of primary importance when we reflect on the danger of such being in the air and the drainage of inhabited areas. That the spores of Bacillus anthracis find their way from the bodies of animals to the surface waters of meadows, and thence into rivers, must be accepted as proved by the researches of Pasteur and Koch,* and our knowledge in this connexion suggests only too plainly what may occur in other cases, thus explaining the observed facts that the microbes of typhoid, cholera, septicemia, &c., do occur in exposed waters; and connecting the presence of other pathogenic forms, known to be cast off in secretions, dejecta, &c., with the suspicions aroused from the washing of milk vessels, &c., with such waters.

It is necessary to bear in mind, however, that although the vista of possibilities here opened out is a real one, most of the bacteriological examinations of water support the conclusion that by far the majority of the Schizomycetes met with in natural waters are harmless, or at least are not capable of producing disease directly in those who drink the waters.

Such conclusions have led to speculations, in different directions, as to why the bacteriological examination of waters has, so far, seldom led to the detection of pathogenic forms, although such waters are exposed to contamination.

Firstly, it is possible that a Schizomycete should lose its virulence or be weakened, or even die, when transferred from a suitable medium into one so thin and innutritious as any ordinary potable water would be; secondly, quite apart from the scarcity of food materials, it requires some reflection to thoroughly grasp how great must be the changes in the circumstances which a given pathogenic form-say, the anthrax bacillus, for argument-meets with when it leaves the living "Recent Bacteriological Research in connection with Water Supply," 'Journ. Soc. Chem. Ind.,' 1887; "The Applications of Bacteriology to Questions relating to Water Supply," "Trans. Sanitary Institute of Great Britain,' vol. 9, 1887); H. A. Nielsen ("The Bacteria of Drinking Water, in particular as regards the Species in the Water Supply of Copenhagen," Copenhagen, 1890; see Ann. d. l'Inst. Pasteur,' 1890, p. 41), Bertschinger (Vierteljahrschr. d. Naturf. Gesellsch.,' vol. 34, 1889, also 'Ann. de l'Inst. Pasteur,' vol. 3, 1889, p. 692), Duclaux ("Le Filtrage des Faux," Ann. de l'Inst. Pasteur,' 1889, pp. 41-56; and "Sur les relations du Sol et de l'Eau qui le traverse," Ann. de l'Inst. Pasteur,' 1889, pp. 172-184); also our Appendix A.

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As to bacteria in ice, snow, and hail, see Prudden ('New York Med. Record,' 1887), Bordoni-Uffreduzzi (Centralbl. f. Bakt. u. Parasitenkunde,' vol. 2, 1887), Janowski (ibid., vol. 4, p. 547), and Schmelck (ibid., vol. 4, p. 545), Fraenkel (Zeitschr. f. Hyg.' vol. 1, pp. 302—314), and Odo Bujwid (* Ann. de l'Inst. Pasteur,' vol. 1, 1887, p. 592).

* Pasteur, 'Bull. de l'Acad. de Médecine,' 1880; Koch, 'Mittheil. a. d. K. Gesundheitsamte,' 1881, p. 49.

body of a sheep and is carried into a stream. In considering this example, the observed facts as to the susceptibility of anthrax to low temperatures should be borne in mind. The great reduction in temperature would alone suffice to impress it with effects very different from those of its previous environment-the tissues of a warmblooded animal-and matters would be made no sinpler by the differences in exposure to the oxygen of the air, the light of the sun, and so forth.*

That such a view is not without foundation is sufficiently proved by recent researches on the action of heat, light, and oxygen on this very bacillus in question.

To take the case of temperature first. It is generally agreed that Bacillus anthracis cannot go on growing and dividing below about 15° C., nor above about 45° C., and that it thrives best at some temperature near 35° C.; it is also agreed that it is markedly susceptible to the presence of free oxygen in its normal development. Although undoubtedly favoured by presence of oxygen, the anthrax bacillus will grow in the presence of only a very small quantity of air, (Liborius, 'Zeitschr. f. Hyg.,' 1, p. 170). Under favourable circumstances, but only if oxygen is present and the temperature fairly high, the bacilli form spores in their interior. This complicates the matter under discussion, for these spores are sometimes capable of remaining uninjured for long periods under conditions which would inevitably kill the vegetative rodlets.

Now Roux+ has lately shown that in a given culture containing these spores some individuals are more resistant than others, and that when germinating it is of importance to a given spore whether it is near the surface of a liquid or deeper down; that at high temperatures, in contact with free atmospheric oxygen, the virulence of a given culture can be attenuated, though no such attenuation results when out of contact with air.

These are by no means all the facts that have to be regarded, however.

Possibly by far the most important of the destructive influences of fresh water on such microbes is that of the change in the conditions of osmosis, which is also entirely substantiated by experiment, and is in harmony with what we know of the physiology of living tissues (see Marshall Ward, "On Some Relations between Host and Parasite," &c., the Croonian Lecture for 1890, Roy. Soc. Proc.,' vol. 47, pp. 393-443, and references to the works of Pfeffer and De Vries therein; also Fischer, “Die Plasmolyse der Bacterien,” in 'Ber. üb. d. Verhandl, Sächs. Gesellsch. Wiss. zu Leipzig,' vol. 1, 1891, pp. 52-74, and Wladimiroff, "Osmotische Vers. an lebenden Bakterien," in 'Zeits. Physik. Chem.,' vol. 7, pp. 529-543).

Roux, "De l'Action de la Chaleur et de l'Air sur les Spores de la Bactéridie du Charbon" (Ann. de l'Inst. Pasteur,' vol. 1, 1887, pp. 392—399).

We ought to deal with this subject very cautiously, for others have stated, and some confuted, this previously; but of course we are not concerned with all the details here.

A large number of investigators, by means of researches first started by Downes and Blunt in 1877,* in this country, and carried on ever since by others, have shown that the action of the sun's rays has to be taken into consideration when dealing with questions of the vitality or rate of growth, &c., of the spores and rodlets of this and other Schizomycetes.

The controversy is too long for full treatment in this report, but the upshot of the whole may be summed up as follows. Certain rays of light, apparently more especially those known as the "chemical rays," so affect the germinating spores of certain bacteria (Bacillus typhosus, Bacteria anthracis), in presence of air, that their growth is inhibited. The presumption is that the solar rays enhance certain oxidation processes in the living protoplasm, but questions also arise in some cases as to possible effects on the nutritive media as well, though Janowski certainly seems to have eliminated these in his cultures of the typhoid bacillus.‡

A second possible view as to the fate of a given species of bacterium when suddenly washed into a stream is that it remains there unaltered, and that the chances are so enormously against its being. detected, or (what, from some points of view, is the same thing) against its finding a suitable nidus in a living animal, that it simply wanders passively in the waste of waters surrounding it for an indefinite period, or until it reaches the sea.

This view also must be faced as one not altogether unsupported by observations, but only on the understanding that the microbe is in the spore stage, or, at least, passes into that condition soon after reaching the water, for the weight of bacteriological experience is distinctly against the probability of a living Schizomycete, in the simple vegetative condition, remaining as such for any length of time, at any rate in such a dilute medium as potable water.

With spores the matter is different. Duclaux found old spores of certain forms which had been kept out of contact with air for several years to be still capable of germination when sown in suitable

Downes and Blunt, 'Roy. Soc. Proc.,' 1877, p. 488, and ibid., 1878, p. 199.

It should be clearly indicated, however, that the evidence goes rather to show that it is insolation which produces these results, and not diffused light. Insolation can have practically no effect in natural waters.

For details as to the action of light on bacteria, consult Raum ("Der Gegenwärtige Stand unserer Kenntnisse ü. d. Einfluss des Lichtes auf Bacterien, &c.," Zeitschr. f. Hyg.,' vol. 6, 1889, pp. 312-368), for full references to literature to date. Then see Pansini (“ Action de la Lumière Solaire sur les Microorganismes,” in Rivista d'Igiene,' 1889; also 'Ann. de l'Inst. Pasteur,' vol. 3, 1889, p. 686); Janowski, ("Zur Biologie der Typhus-bacillen," in 'Centralbl. f. Bakt. u. Parositenk.,' 1890, Nos. 6-8); F. Elfving, 'Studien über die Einwirkung des Lichtes auf die Pilze,' Helsingfors, 1890, 139 pp. and 5 plates-deals more especially with fungi proper—and our Appendix A.

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media,* and it is well known what extremes of temperature, &c., spores can withstand. At the same time, since the rule is that a spore germinates in even dilute solutions, when transferred thither, in presence of oxygen and if the temperature rises, it may be regarded as probable that, for aërobic bacteria at any rate, the changing conditions in a river, &c., will prevent its remaining merely passiveall available evidence is rather in favour of its either growing or else dying if it cannot adapt itself to the circumstances, although the death of spores may be delayed for many months and possibly even longer.

Indeed, recently, strong evidence has been produced, showing that pathogenic microbes may sink to the bottom of lakes and rivers and there remain in a living state, amongst the sediment or mud, for very long periods of time, until in fact, some flood or other disturbance causes them to become once more suspended in the water, when they may be carried by a stream or current to another place. It is obvious that this hitherto but little recognised factor is of the very highest importance in connexion with the supply of water from rivers subject to objectionable pollution.†

A third view is possible, viz., that the Schizomycete finds the new environment at least not unsuited to its immediate requirements, and that it grows and multiplies more or less successfully in the large mass of water.

This unquestionably happens with some forms, which, as we have seen, are so well adapted for life in rivers, ponds, and even pipes, that they have long been known as aquatic species.‡ As has been stated, and will be seen more clearly shortly, however, this is also true, to a limited extent, of many forms, including certain pathogenic species, which are only met with in natural waters as intruders; they are able to maintain themselves alive for variable periods, and then usually succumb.

Before passing to this part of the subject, we wish to remark upon the method for a long time employed in the bacteriological examination of water, and on some of the general results obtained.

Since 1881 it has been almost universally the custom to employ the gelatine-plate cultures as devised by Koch. A measured small quantity of the water to be examined is added to the nutrient gela

Quoted by Roux ('Ann. de l'Inst. Pasteur,' vol. 1, 1887, p. 392).

+ Lortet and Despeignes, "Recherches sur les Microbes Pathogènes des Eaux Potables distribuées à la Ville de Lyon" (Rev. d'Hygiène,' 12, 1890, No. 5) ; also Lortet, "Die pathogenen Bakterien d. tiefen Schlammes im Genfer See" (Centralbl. f. Bakter.,' 9, 1891, p. 709).

This term is, of course, not quite accurate, in view of the fact that all Schizomycetes must have water to grow; and are, indeed, descended from aquatic forms -lower Algæ.

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