[21] A natural chlorate of potassium, generally of volcanic origin.—ED.
[22] That is to say, he reflected the beam of polarized light by a mirror placed at that angle. See Turpain, Leçons élementaires de Physique, t. ii. p. 311, for details of the experiment.—ED.
[23] It will no doubt be a shock to those whom Professor Henry Armstrong has lately called the "mathematically-minded" to find a member of the Poincaré family speaking disrespectfully of the science they have done so much to illustrate. One may perhaps compare the expression in the text with M. Henri Poincaré's remark in his last allocution to the Académie des Sciences, that "Mathematics are sometimes a nuisance, and even a danger, when they induce us to affirm more than we know" (Comptes-rendus, 17th December 1906).
[24] See footnote 3.
[25] I.e. 10,000 metres.—ED.
[26] By this M. Poincaré appears to mean a radiometer in which the vanes are not entirely free to move as in the radiometer of Crookes but are suspended by one or two threads as in the instrument devised by Professor Poynting.—ED.
[27] See especially the experiments of Professor E. Marx (Vienna), Annalen der Physik, vol. xx. (No. 9 of 1906), pp. 677 et seq., which seem conclusive on this point.—ED.
[28] M. Sagnac (Le Radium, Jan. 1906, p. 14), following perhaps Professors Elster and Geitel, has lately taken up this idea anew.—ED.
[29] At least, so long as it is not introduced between the two coatings of a condenser having a difference of potential sufficient to overcome what M. Bouty calls its dielectric cohesion. We leave on one side this phenomenon, regarding which M. Bouty has arrived at extremely important results by a very remarkable series of experiments; but this question rightly belongs to a special study of electrical phenomena which is not yet written.
[30] A full account of these experiments, which were executed at the Cavendish Laboratory, is to be found in Philosophical Transactions, A., vol. cxcv. (1901), pp. 193 et seq.—ED.