668 EEFORT— 1901. 



density will tecome less ; and large regions will quickly become void or nearly 

 void of atoms. These large void regions would extend so as to completely sur- 

 round regions of greater density. In some part or parts of each cluster of atoms 

 thus isolated, condensation would go on by motions in all directions not generally 

 convergent to points, and with no perceptible mutual influence between the atoms 

 until the density becomes something like lO"'' of our ordinary atmospheric density, 

 when mutual influence by collisions would begin to become practically effective. 

 Each collision would give rise to a train of waves in ether. These waves would 

 carry away energy, spreading it out through the void ether of infinite space. The 

 loss of energy, thus taken away from the atoms, would reduce large condensing 

 clusters to the condition of gas in equilibrium ^ under the influence of its own 

 gravity only, or rotating like our sun or moving at moderate speeds as in spiral 

 nebulas, &c. Gravitational condensation would at first produce rise of tempera- 

 ture, followed later by cooling and ultimately freezing, giving solid bodies ; 

 collisions between which will produce meteoric stones such as we see them. We 

 cannot regard as probable that these lumps of broken-looking solid matter (some- 

 thing like the broken stones used on our macadamised roads) are primitive forms 

 in which matter was created. Hence we are forced, in this twentieth century, to 

 views regarding the atomic origin of all things closely resembling those presented 

 by Democritus, Epicurus, and their majestic Roman poetic expositor, Lucretius. 



2. A Discussion on Glass used for Scientific Purposes. 

 0]}ened by a Paper by Dr. R. T. Glazebrook, F.R.S. 



3. The Brush Grating and the Law of its Oj)tical Action, 

 By John Kerr, LL.B., F,R.S. 



Pure water is rendered slightly hazy by holding in suspension a small quantity 

 of chemically precipitated and invisibly fine particles of FejO, ; this liquid placed 

 in a uniform and moderately strong magnetic field gives the best known example 

 of the Brush grating. The water is luiderstood to bo traversed throughout its 

 mass by a set of invisibly fine filaments of solid particles, all straight and parallel. 

 "When this medium is examined in the polariscope the vibrations transmitted are 

 always perpendicular to the filaments. 



The action of the Brush grating comes out ill experiment as twofold : (1) a 

 negative double refraction with filament for optic axis ; (2) a selective absorption 

 of the extraordinary ray. The phenomena are quite regular, and as pure as any 

 that are given by good crystals, but upon a comparatively small scale of intensity. 

 The simplest statement of the law of the action is that when light passes through 

 the Brush grating the Fresnel vibrations parallel to the filaments are the most 

 absorbed, and those perpendicular to the filaments the most retarded. 



It is interesting, and may be iiseful, to compare the new medium with the 

 numerous media known in optics as the coloured birefringent crystals ; and also 

 with Hertz's grating of parallel wires, used as a transmitter and absorber of 

 electric waves. 



4. Tlie Effect of Errors in Ruling on the Appearance of a Diffraction 

 Grating. By H. S. Allen, M.A., B.Sc. 



If a spectroscope is adjusted to view a single spectral line, and the eye-piece of 

 the observing telescope is removed, the diftraction grating is seen illuminated by 

 monochromatic light ; but in general the image is crossed by a number of dark 

 bands parallel to the rulings on the grating. The bands may be better studied by 

 focussing the observing telescope on the surface of the grating instead of on the 



' Homer Lane, American Journal of Science, 1870, p. 57; Sir W. Thomson, 

 Phil. Mag., March 1887, p. 287. 



