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.
Prof. Crookes has at length announced a theory in explanation of the movements exhibited by the remarkable "light mill" of his invention. He says: "The evidence afforded by the experiments is to my mind so strong as almost to amount to conviction, that the repulsion resulting from radiation is due to the action of thermometric heat between the surface of the moving body and the case of the instrument, through the intervention of the residual gas. This explanation of its action is in accordance with recent speculations as to the ultimate constitution of matter, and the dynamical theory of gases." The most refined means for exhausting the air from the glass bulb which contains the suspended vanes of the radiometer leave, and if they were to be carried to absolute mechanical perfection, would still leave a certain amount of gas in it. But Dr. Crookes has carried this attenuation so far that the number of gas molecules present can no longer be considered as practically infinite. Nor is the mean length of their paths between their collisions any longer very small compared to the size of the bulb. The latest use to which the radiometer has been put was to test the viscosity of gases at decreasing pressures. The glass bulb was furnished with a stopper lubricated with burnt rubber. This was fixed and carried a fine thread of glass which is almost perfectly elastic. To the end of this thread hung a thin oblong plate of pith to which a mirror was attached. The glass stopper being fixed, and the bulb capable of rotation through a small angle, it is evident that when the bulb is rotated the pith ball will remain at rest except as it yields to the friction of the air moved by the bulb. It does move, swinging a certain distance and then back, like a pendulum. The amount of this movement is carefully observed by a telescope, and recorded for five successive beats. As the pith and glass fibre form a torsion pendulum, it is evident that these beats will gradually die down in consequence of the resistance of the air. By exhausting the air to various degrees of rarity, it was proved that Prof. Clerk Maxwell's theory, that the viscosity of a gas is independent of its density, is correct. The logarithmic decrement of the first five oscillations (that is, the decrease, oscillation by oscillation, of the logarithm of the arc through which the pith vanes swing), was found to be nearly the same when the air was almost exhausted as when it was at its natural pressure, proving that its viscosity remained nearly equal for all pressures. Only in the exceptionally perfect vacuum referred to above did this logarithmic decrement sink to about one-twentieth of what it had commenced with. Repulsion of the vane by the action of light commences when this decrement is one-fourth of what it was before the exhaustion of air began. As the rarity of the air within the bulb increases the force of this repulsion begins to diminish, like the logarithmic decrement, and when the latter has sunk to one-twentieth the former has fallen off one-half. All these and other facts previously obtained prove that the action of light is not direct, but indirect; and Dr. Crookes has, after repeatedly refusing to consider hasty judgments, in consequence come to the conclusion stated above, that the rotation of the light mill is the result of heat. This decision accords with the opinion of other observers. The radiometer has already entered the field of industrial science, and is used to measure the duration of exposure of photographic plates. De Fonvielle has made with it a new determination of the sun's thermometric power. He made a spectroscope with a graduated screen, which permitted the amount of light that entered the apparatus to be graduated at will. In the path of the beam he placed a radiometer, and by comparing its action in the graduated light ray, and in the light of a standard oil lamp, burning 42 grammes (11.3 ounces Troy) per hour, he found that at 4 o'clock, on June 4, 1876, the radiating force of the sun was equal to 14 lamps placed 25 centimetres (10 inches) from the radiometer.
TEMPERED GLASS IN THE HOUSEHOLD.
The "tempered glass," which has made the name of M. de la Bastié, its discoverer, so well known, does not prove to be always manageable. It was to have the strength of metal, and not shiver with changes of temperature. But an English lady has found that it sometimes has precisely the contrary characteristics. She purchased twelve globes for gaslights, and they were made in the manufactory of M. de la Bastié himself. But one night, after the gas had been extinguished for exactly an hour, one of the globes burst with a report, and fell in pieces on the floor, leaving the bottom ring still on the burner. These pieces, which were of course found to be perfectly cold, were some two or three inches long and an inch or so wide. They continued for an hour or more splitting up and subdividing themselves into smaller and still smaller fragments, each split being accompanied by a slight report, until at length there was not a fragment larger than a hazel nut, and the greater part of the glass was in pieces of about the size of a pea, and of a crystalline form. In the morning it was found that the rim had fallen from the burner to the floor in atoms. In all these phenomena the behavior was that of unannealed glass, of which so many curious performances have been related.