As explanation of this paradoxical dark photography--or whatever it is--two theories may be advanced. The one--favoured by Professor Russell--is that all “active” substances give off _vapours_ able to act on a photographic plate. In support of this may be urged the fact that the interposition of glass prevents the making of dark pictures. But on the other hand it must be remembered that celluloid and sheet-gelatine, also air-tight substances, are able to store up light and to give it out again. It is well known among photographers that to allow sunlight to fall on the inside of a camera is apt to have a “fogging” effect on a plate that is exposed in the camera afterwards, though the greatest care be taken to keep all external light from the plate. But here the glass again presents a difficulty, for if this were a case of reflected light, glass would evidently be _less_ obstructive than opaque vegetable parchment or gutta-percha. SOLAR MOTORS. One day George Stephenson and a friend stood watching a train drawn by one of his locomotives. “What moves that train?” asked Stephenson. “The engine,” replied his friend. “And what moves the engine?” “The steam.” “And what produces the steam?” “Coal.” “And what produces coal?” This last query nonplussed his friend, and Stephenson himself replied, “The sun.” The “bottled sunshine” that drove the locomotive was stored up millions of years ago in the dense forests then covering the face of the globe. Every day vegetation was built by the sunbeams, and in the course of ages this growth was crushed into fossil form by the pressure of high-piled rock and débris. To-day we cast “black diamonds” into our grates and furnaces, to call out the warmth and power that is a legacy from a period long prior to the advent of fire-loving man, often forgetful of its real source. We see the influence of the sun more directly in the motions of wind and water. Had not the sun’s action deposited snow and rain on the uplands of the world, there would be no roaring waterfall, no rushing torrent, no smooth-flowing stream. But for the sun heating the atmosphere unequally, there would not be that rushing of cool air to replace hot which we know as wind.