The presence of water and a suitable temperature are indispensable conditions of the oxidizing process of decay, just as they are necessary to putrefaction and fermentation. The sides of ponds and ditches being covered by water during the winter months, in the spring the air becoming warmer and drier, the water diminishes, the decay of vegetable seeds, plants, and all woody fibres enter now into putrefaction, communicating the process to each other, and by the transmission of decomposition from one particle to another, a great number of plants give out various gases to the atmosphere while decaying upon mud, rise into the air, meeting other gases, and then, floating about, they compose and decompose each other. Hence the bad odour from the mud-banks of the Thames, near the outfalls of the sewage. NOTE 8. I have known fevers cured by a change of the sleeping room from the south to the north aspect, and still more readily by removing from one side of the street to the other. All should avoid dwelling near canals, ponds, or ditches habitually covered with a white froth; this is formed, in fact, of gases rising through humus swimming on the water, and contains living beings as well as fermentable substances. It is important to men who work and sleep in the same house to have the day or working-rooms to the north, where the sun never enters, and to sleep in a room to the east or south. A room to the west, looking to the west, is not healthy, particularly in summer months, being the hottest in the evening. Gnats, moths, and flies collect there, and are at least harassing, if not hurtful, particularly to infants. No person not a native of a marshy country should travel overland in the evening; dew causes a strong action in vapours, mists, &c. Invalids and soldiers after fatigue, should halt in the daytime, and march in the evening, to avoid being chilled. NOTE 9. A sure remedy against the malaria of ditches, ponds, &c., is to fill the water-courses with water; never suffer them to be so far dried up that the spawn of living creatures may attach itself to the sides of grass, bushes, &c., and afterwards to dry and spread about like the seeds of flowers, in the environs. The mud which is left exposed to the air gives out, on drying, various gases, which being mixed with the fossils of the mud, contaminate the air, and are breathed by the people in the neighbourhood. A circular drain, having a double current, well understood by the hydraulic engineers of Holland, is the kind of drain I prefer. THE END. MR. RENSHAW’S PUBLICATIONS. A TREATISE ON HOOPING COUGH, with its Treatment by a New Remedy. By George D. Gibb, M.D. Fcap. cloth. Price 7_s._ THE DISEASES OF THE FŒTUS IN UTERO. By Henry Madge, M.D. Fcap. 8vo. cloth. Price 5_s._ ON THE EPIDEMIC FEVERS OF SIERRA LEONE. By Alexander Bryson, M.D. 12mo. cloth. Price 4_s._ FRORIEP ON THE APPLICATION OF ELECTRO-MAGNETISM: translated by Dr. Lawrance. 8vo. cloth. Price 5_s._