THE PHOTOPHONE IN RESEARCH.
THE elegant research of Professor Tyndall, which we publish in the present number, will well repay the careful attention of our readers. It is of interest, not only on account of the very complete confirmation of results previously obtained by this physicist, but also on account of the novel method employed, and the promise this gives of wide utility. The photophone is barely six months old, but these experiments show that it already has a large field of usefulness before it, and it is, perhaps, not too much to expect that it will prove to be one of the most delicate instruments at the command of the physicist. The experiments are further interesting for the very conclusive demonstration they afford of the causes to which the action of the instrument is due. From the first, Professor Tyndall states, he was convinced that the sounds given out by bodies upon which the intermittent beam of light impinged were due to their expansion and contraction under the influence of radiant heat, and this opinion is most fully borne out by the results obtained. The experiments, while showing the great delicacy of this beautiful instrument of Professor Bell, also incidentally show that some of the expectations with regard to it are unfounded. One of these is, that with it sounds upon the sun may be heard. The fallacy of this has been recently pointed out, and the arrangement of the apparatus adopted by Professor Tyndall clearly exhibits it. It consists in assuming that the sound given out by the absorptive body is the reproduction of a previous sound, while in reality all that is necessary is that the impinging beam be intermittent—its variations may be produced in any manner whatever.
Of the results of previous experiments confirmed by this later research, the most important are those regarding the behavior of dry air and the vapor of water toward radiant heat. By a long series of beautiful and refined experiments, Professor Tyndall had shown that the former was perfectly transparent to such heat, while water-vapor was a powerful absorbent of it. These results have been disputed by other experimenters, and it needed, to definitely settle the controversy, some more delicate method of testing these substances than that furnished by the instruments heretofore at command. This has been supplied by this latest acquisition of science, and the first use of it appears to fully sustain Professor Tyndall's position.
BOOK PHYSIOLOGY ABROAD.
We often hear subdued expressions of doubt as to the quality of the physiological teaching prevalent in girls' schools. It is intimated that the knowledge the pupils get upon this subject is generally of a very loose and vague sort, so as to be but of little practical use. It is objected to what girls learn about in their physiological studies, that it is not entitled to be called knowledge at all—that is, they do not really know what they are studying about, but only remember certain statements as well as they can, while the information they get is not of a kind fit to be used. Whatever may be the fact in regard to our own schools, it is pretty certain that the physiology taught to girls in some of the English schools is marked with all the bad qualities sometimes ascribed to our own.
The London "Globe" gives a ludicrous illustration of the results of phys-
