Affinity Illustrated.—The affinity or power of uniting one substance with another is so great, that, were it not for living plants and animals, each element of the world would soon seek out its fondest ally, and these being united, there would quickly be an end to any further chemical change of matter on the face of the earth. The vital power, however, of living plants and animals is constantly undoing what the inorganic or non-vital materials are ever consummating ; the very few native or natural elements that are found by man show how this power has already done its work. Man never finds iron, phosphorus, potassium, carbon, and a host of other materials, in their primitive state, but always combined with some other of the elements ; and it is his ingenuity and chemical knowledge which break them up and separate them, giving us iron for the plowshare, phosphorus for the match, and many other necessaries of civilized life. The laws of affinity are best illustrated by the events of every-day life, such as the burning of a candle, the decay of wood, the change of lime into chalk, and the rusting of iron. Tallow at the ordinary temperature has but little affinity for the oxygen in the air ; it has, however, sufficient affinity for it, and gradually changes or becomes, as we say, rancid. The higher the temperature the greater is this affinity. If tallow be thrown on to hot iron, as in a frying pan, then a further change is noticed m the powerful odorous bodies produced. At a burning heat, however, the affinity of the oxygen of the air and the components of the tallow is so great that the whole disappears in invisible gases. Wood shows a similar action, according to the temperature it is exposed to. If air, wood, and water be exposed together, their natural affinities are sufficient to sap " the heart of oak " in five years ; and if heated to the combustion point, this change takes place in a few minutes. If we make a paste of lime and water, and spread it on a tile, and then expose it to the air, in less than a month the carbonic acid which is in the air will unite with the lime and produce chalk. Now if vinegar be poured on this chalk an effervescence is produced by the escape of the carbonic acid ; the vinegar (acetic acid) having a greater affinity for the lime than the carbonic acid, throws out the latter. Iron stone as it is dug from the mine is little else than rust (or oxyd) of iron—that is, oxygen from the air united with the metal. The smelter's business is to make the oxygen in the metallic rust unite with the coal, which it readily does at a furnace heat, and thus he shows us how he can break up that affinity which has hidden the bright metal from mortal gaze since the world began. The want of the knowledge of the laws of affinity betokens savage life ; on the contrary, a thorough comprehension of affinity indicates a high state of civilization.
This article was originally published with the title "Laboratory—No.4" in Scientific American 13, 33, 262 (April 1858)