Page:Popular Science Monthly Volume 11.djvu/171

trace them to a common source. Observations at Kew for a series of years have detected that the proximity of Mercury and Venus to the sun seems to control the size and direction of the solar spots; and therefore it appears that these planets mediately through the sun cause our auroras and magnetic storms. There is also reason to believe that the production of sun-spots diminishes by absorption the actinic rays, while the thermal ones are not noticeably affected; for to the actinic rays the chemical or ripening effect is due. Years of minimum sun-spots have been found to coincide very nearly with the good wine years in Germany. At first sight we are startled by the supposition that a planet like Venus, which comes nearer to the earth than it ever does to the sun, should in any way be accountable for such enormous manifestations of energy as those which occur over the sun's surface. But the wonder will disappear if we bear in mind that there may be two kinds of causes or antecedents. Thus, we say that the blacksmith is the cause of the blow which his hammer strikes the anvil, and here the strength of the blow depends upon the strength of the smith. But we may likewise say that the man who pulls the trigger of a gun or cannon is the cause of the motion of the ball, and here there is no relation between the strength of the effect and that of its cause. Now, in whatever mysterious way Venus and Mercury affect the sun, we may be sure it is not after the fashion of the blacksmith: they do not deal him a violent blow producing all this enormous effect, but they rather pull the trigger, and immediately a very great change takes place. And, in passing, we are here taught how involved the relations of the parts of the universe may be. Two planets whose direct influence on the earth by their gravitation and light is quite inconsiderable, yet, by their indirect effects through their action on the sun, produce very marked and varied results on the surface of our globe. Many laboratory experiments, on a small scale, illustrate the potency of initiatory forces. A pail of water, maintained in great stillness, may be gradually chilled several degrees below the common freezing-point, when the formation of a cake of ice instantly follows from a slight shake. A clean glass vessel may be filled with water and slowly brought to a temperature in excess of the ordinary boiling-point, and a feeble shock is all that is needed for the prompt liberation of a large volume of steam. Crystallization offers similar results. A supersaturated solution of a salt may long remain in the liquid state until a crystalline fragment thrown in instantly serves as a nucleus for extensive solidification. When such a solution contains two salts, which begin to crystallize about the same temperature, when a solid fragment of each kind is thrown into the liquid, it picks out its kindred without the slightest error, and grows thereby. A new surgical method for covering a wound with skin employs as centres of growth tiny morsels of skin supplied from elsewhere. These gather together elements akin to themselves, and soon