I. On the thermal resistance of liquids

to the plane of reflection, when the suspended particles became finer, and therefore the beam more nearly continuous.” Through the courtesy of its owner, I have been permitted to see and to experiment with the piece of plate glass above referred to. Placed in front of the electric lamp, whether edgeways or transversely, it discharges bluish polarized light laterally, the colour being by no means a bad imita­ tion of the blue of the sky. Prof. Stokes considers that this deportment may be invoked to decide the question of the direction of the vibrations of polarized light. On this point I would say, if it can be demonstrated that when the particles are small in comparison to the length of a wave of light, the vibrations of a ray reflected by such particles cannot be perpendicular to the vibra­ tions of the incident ligh t; then assuredly the experiments recorded in the foregoing communication decide the question in favour of Fresnel’s assumption. As stated above, almost all liquids have motes in them sufficiently nu­ merous to polarize sensibly the light, and very beautiful effects may be obtained by simple artificial devices. When, for example, a cell of dis­ tilled water is placed in front of the electric lamp, and a slice of the beam permitted to pass through it, scarcely any polarized light is dis­ charged, and scarcely any colour produced with a plate of selenite. But while the beam is passing through it, if a bit of soap be agitated in the water above the beam, the moment the infinitesimal particles reach the beam the liquid sends forth laterally almost perfectly polarized light; and if the selenite be employed, vivid colours flash into existence. A still more brilliant result is obtained with mastic dissolved in a great excess-of alcohol. The selenite rings constitute an extremely delicate test as to the quantity of motes in a liquid. Commencing with distilled water, for example, a thickish beam of light is necessary to make the polarization of its motes sensible. A much thinner beam suffices for common water; while with Briicke’s precipitated mastic, a beam too thin to produce any sensible effect with most other liquids, suffices to bring out vividly the selenite colours.