I. W. Heysinger in a recent address on “The Future Water Supply of Philadelphia,” delivered at the Hahnemann Medical college in that city, spoke as follows on the subject of disease germs, and sand filtration:

The old idea that sand filtration was due to the sand has been abandoned. We now know that bacteria will pass through sand just as thin ,mud will pass through a bed of brickbats. An organic slime is formed upon the upper surface of the sand, and this thin pellicle of slime, a sort of gelatinous protoplasmic screen, takes the water from above with its bacteria, and the water, minus its bacteria in large part, exudes beneath, and then runs down through the different layers of sand and gravel in its outlet. The typhoid germs are, in a large degree, caught, and so are the numerous innocuous and, in many cases, beneficent bacilli, which by their presence prevent sporadic infection, and the water is largely deprived of its self-preservative properties. It is deadened to that extent, and is deprived of the living organisms which powerfully aid in preventing subsequent deterioration. Then if this thin layer of slime is broken away,at any point or upon any surface.it is undiscovered, and the whole security is at once gone—the fact is evidenced only by an epidemic. The surface of the sand and the slimes require to be scraped off at short intervals,during which time the water is drawn off. The debris of these slimes will then be left commingled with the sand to produce subsequent deterioration, and possibly infection of a virulent character, should the pathogenetic bacteria be still alive. We must not forget that the city of Paris, up to this day. is not satisfied to adopt sand filtration, but is experimenting with other systems.

Hut a still greater danger from filtration, as applied to positively and decidedly contaminated water, is to be found in the very terms of the problem for the water supply of a large city. The delicate manipulations and careful attention requisite to insure perfect results are continuous of necessity. ‘ Familiarity breeds contempt.’ These processes are invisible to the people: the agents in whose charge they are are unknown personally: it is part of a methodical system in which the dangers appear far off and exaggerated. Workmen become careless; that is why gunpowder and nitro glycerine factories explode; but in this case a careless workman Can explode a whole city and go scot-free himself. For. as Colonel l.udlow says, ’The smallest possible drop of water would be a home and nidus for millions of them,and they would pass thousands abreast through the pores of any filter adapted to the flow of water.

It is not to be denied that carefully selected instances show the power of this film of organic slime, overlying a quiescent bed of sand, to detain, in large part, the typhoid germs and other germs which ought not to be detained. Hut this film is very delicate—a slightly increased flow, a puff of the breath, even, will break it up, and, when broken.it becomes a veritable fountain of infection.

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