The principles I have laid down afford a satisfactory explanation of the circumstances, that absence of drainage promotes the prevalence of cholera, and that it flourishes better on a clay soil than on primitive rocks, sandstone, or gravel. Without drainage, the refuse of the population permeates the ground, and gains access to the pump-wells. Merthyr Tydvil, with 52,863 inhabitants, is entirely without drainage, and the people derive their supply of water from pump-wells. This place has suffered severely from cholera in every epidemic. In 1849 there were 1,682 deaths from this disease, being 234 to each 10,000 inhabitants,—a rate of mortality as high as in Hull and certain of the south districts of London, where the morbid poison of cholera was distributed by the steam-engines of the water companies. The primitive rocks, sandstone, and gravel, generally cause the purification of the water by the separation or oxidation of organic matters, whilst clay does not exert this salutary influence to the same extent. Since the latter part of 1848, when I first arrived at my present conclusions respecting the mode of communication of cholera, I have become more and more convinced that many other diseases are propagated in the same way. When the plague visited this country, it was most fatal in London, York, Winchester, and certain other towns having a river of fresh water passing through them. It resembled cholera also in being twice as fatal in the districts on the south of the Thames as in those on the north. The following passage from Stow’s “Survey”, published in 1633, shows the way in which Southwark was supplied with water about the time of the great visitations of plague: “Southwark useth chiefly the water of the Thames, that falls into a great pond at St. Mary Overies, that drives a mill called St. Saviour’s Mill, the owner whereof is one Mr. Gulston. The revenue thereof is supposed by some to be worth 1,300_l._ a year.” [Sidenote: MODE OF PROPAGATION OF THE PLAGUE.]