amoeba the shell-protected rhizopods were no doubt evolved, and it is a curious sight to observe them swallowed, home and all, to be digested out of their house. It was also interesting to observe the cannibal amoeba swallowing one of its own kind and appropriating its structure to its own use, just as we might do the contents of an egg. The amoeba verrucosa he describes as remarkable for its sluggish character, and in appearance reminds one of a little pile of epithelial scales or a fragment of dandruff from the head. It is oval or rounded, transparent, and more or less wrinkled, or marked with delicate, wavy lines. THE SOURCES OF POTABLE WATER. In the British Social Science meeting, Mr. Latham, a civil engineer of London, brought up the question of water supplies and endeavored to find rules for the guidance of water engineers in those apparently contradictory facts which the observation of recent years has produced so abundantly. It has been generally considered that water which has received the sewage of large populations must be unfit for domestic use; but careful investigation would show that when such polluting matter has been passed into a river, and exposed to the influence of light, vegetation, etc., it becomes innocuous. This is shown by the good health enjoyed by the inhabitants of London, which place receives its supply chiefly from the Thames and the Lea, both of which rivers receive a considerable amount of sewage pollution. The author instanced Wakefield, Doncaster, and Ely as towns that draw their supplies of water from sources into which sewage matter enters, and yet whose inhabitants are healthy. The cholera epidemic at Newcastle-on-Tyne in 1853 was supposed to have been caused by the use of polluted Tyne water, and yet it was clearly ascertained that disease was much more rife among those persons who used local well water. These facts, which have often been quoted, were not favorably received by the audience, who greeted with laughter Mr. Latham's assertion that water into which sewage matter has entered can be purified by a short exposure to the air. That statement may be too strong; but there is acknowledged truth in the author's main point. He considered it was clearly proved that water derived from underground sources, or from which light and air have been excluded, is impure, and consequently unfit for domestic use. Universal testimony showed that decaying matter easily found its way into underground sources of supply. Well water may become seriously contaminated by the slow steeping of noxious matters, and be less wholesome than the water of a running stream that receives much larger quantities of impurity. THEORY OF THE RADIOMETER.