It is amusing to recall that, at the time when these results were first published, the public, instead of being reassured by these facts, were greatly alarmed, and it is a matter of history that the mere demonstration of the presence of micro-organisms in drinking-water caused a fall in the price of several of the water companies' stocks! These investigations, which have since been confirmed by others both in this country and on the Continent, have clearly shown, then, that sand-filtration, when carefully carried out, offers a most remarkable and obstinate barrier to the passage of microbes, and there was every justification in presuming that if disease organisms should at any time be present in the raw untreated water, they would also undergo a similar fate, as there was no reasonable ground for supposing that they would behave any differently from the ordinary harmless water bacteria. But this was a hypothesis only, and, however satisfactory experiments in this direction made in the laboratory might prove, there was always the uncertainty attaching to a fact which had not passed through the ordeal of practical experience. The answer to this searching and all-important question has been furnished in the most conclusive manner by the history of the cholera epidemic in Hamburg and Altona respectively in the year 1892. These two cities are both dependent upon the River Elbe for their water-supply, but whereas in the case of Hamburg the intake is situated _above_ the city, the supply for Altona is abstracted below Hamburg _after it has received the sewage of a population of close upon 800,000 persons_. The Hamburg water was, therefore, to start with, relatively pure when compared with that destined for the use of Altona. But what was the fate of these two towns as regards cholera? Situated side by side, absolutely contiguous, in fact, with nothing in their surroundings or in the nature of their population to especially distinguish them, in the one cholera swept away thousands, whilst in the other the scourge was scarcely felt; in Hamburg the deaths from cholera amounted to 1,250 per 100,000, and in Altona to but 221 per 100,000 of the population. So clearly defined, moreover, was the path pursued by the cholera, that although it pushed from the Hamburg side right up to the boundary line between the two cities, it there stopped, this being so striking that in one street, which for some distance marks the division between these cities, _the Hamburg side was stricken down with cholera, whilst that belonging to Altona remained free_. The remarkable fact was brought to light that in those houses supplied with the Hamburg water cholera was rampant, whilst in those on the Altona side and furnished with the Altona water not one case occurred.