From triggers electrical we now pass to triggers chemical. A gun may be charged with powder and remain for years perfectly at rest until a touch on the trigger explodes the powder with tremendous effect. The example is typical: nature and art abound with cases where a little energy, rightly directed, controls energy vastly, perhaps infinitely, greater in quantity. Often in a chemical compound the poise of attraction is so delicate that it may be disturbed by a breath, or by a note from a fiddle, as when either of these induces iodide of nitrogen to explode. A beam of light works the same result with a mixture of chlorine and hydrogen. One of the most familiar facts of chemistry is that a fuel, such as coal, may remain intact in air for ages. Once let a fragment of it be brought to flaming heat and all the rest of the mass will take fire too. Iron has a strong affinity for oxygen, but for union there must be at the beginning some moisture with the gas; the same is true of carbon. A burning jet of carbon monoxide may be extinguished by plunging it into a jar of dried oxygen. Gases from the throat of a blast furnace, at a temperature of 250° to 300° Centigrade, are not inflammable in the atmosphere until the air is moistened by steam or otherwise. Then in a flash combustion begins in earnest. In photography we meet with similar facts: violet rays may begin an impression which yellow light can finish and finish only. Vulcanite is transparent to red and infra-red rays which, although without action upon an unexposed plate, are capable of continuing the action of actinic rays upon a plate which has been exposed for a very short time. Why Weather is Uncertain.