hot-water plate, of course full of boiling water; upon this warm surface pour about half an ounce of the best ether; and, after waiting a few minutes until the whole is converted into vapour, take a syphon made of half-inch pewter tube, and warm it by pouring through it a little hot water, taking care to allow the water to drain away from it before use. After placing the syphon in the tin vessel, a light may be applied to the extremity of the long leg outside the tin vessel, to show that no ether is passing over until the air is sucked out as with the water-syphon; and after this has been done, several warm glass vessels may be filled with this heavy vapour of ether, which burns on the application of flame. Finally, the remainder of the vapour may be burnt at the end of the syphon tube, demonstrating in the most satisfactory manner that the vapour is flowing through the syphon just as spirit is removed by the distillers from the casks into cellars of the public-houses. (Fig. 74.) [Illustration: Fig. 74. A. Tin vessel containing the hot-water plate, B, upon which the ether is poured. C. The syphon. D. Glass to receive the vapour. E. Combustion of the ether vapour in another vessel.] [Page 59] Before dismissing the important subject of specific gravity (or, as it is termed by the French _savants_, "density"), it may be as well to state that astronomers have been enabled, by taking the density of the earth and by astronomical observations, to calculate the gravity of the planets belonging to our solar system; and it is interesting to observe that the density of the planet Venus is the only one approaching the gravity of the earth:-- The Earth 1.000 The Sun .254 The Moon .742 Mercury 2.583 Venus 1.037 Mars .650 Jupiter .258 Saturn .104 Herschel .220 CHAPTER VI. ATTRACTION OF COHESION.