Two independent sets of observations of the electro-magnetic rotation of the plane of polarisation in gases have recently been made—one by MM. Kundt and Röntgen in Strassburg, the other by M. Henri Becquerel, of Paris. The details of the systematic and elaborate research of the former are given in Wiedemann's Annalen. The general result was arrived at, though without sufficient precision to formulate the mathematical law of dependence, that those gases which have the highest indices of refraction possess the greatest rotatory power under magnetic strain. The gases examined—air, oxygen, nitrogen, carbonic oxide, carbonic dioxide, coal-gas, ethylene, and marsh-gas, gave a rotation agreeing in sense with that of the magnetising current. The authors also speculate upon the probability that the plane of polarisation of the atmosphere would be found to be rotated under the influence of terrestrial magnetism, and calculate from their results that a thickness of no less than 253 kilometres of air would be necessary to produce a rotation of 1° in a northeasterly azimuth. M. Becquerel approached the subject from a completely different point of view. Some months ago, when examining the vapour of carbon disulphide, he had found an abnormal apparent difference in its optic rotatory power according to the position of the tube in which it was examined. While studying another matter, however, a flood of light was thrown on this observation. In the endeavour to determine as exactly as possible the position occupied by the plane of polarisation of the sky with respect to the position of the sun, he designed an instrument by means of which the traces of the plane containing the line of sight and passing through the sun could be compared with those of ihe plane of polarisation as observed in a Savart polari cope. With this instrument it was soon found that, contrary to what has always been hitherto supposed, these planes do not coincide with one another, but that the angle between them may even exceed 6 degrees. The plane of polarisation is, moreover, always nearer the horizon than the sun, while the angle between the planes presents diurnal maxima and minima, a point of extreme interest. The electro-magnetic rotation of the plane of atmospheric polarisation is distinctly proved by the following observation:—At noon the position of the sun is such as to produce an illumination of the sky symmetrical with respect to the meridian, which ought therefore to coincide with the plane of polarisation; but as a matter of fact the coincidence of the two planes does not occur at noon, but at a later hour, so that the plane of polarisation has obviously been rotated through a certain angle. This rotation corresponds with the results obtained by direct observation by M. Becquerel upon the magnetic rotatory power of air, as regards both the magnitude and the sense of the rotation. The existence of rotatory power in gases is thus confirmed from a most unexpected source.