The habit of extending analogies beyond their legitimate application was a source of confusion in the early ages of science. Most of the superstitions of primitive religion, of astrology, and of alchemy, arose from this source. A good example is the extension of the metaphor in the words _generation_ and _corruption_: words in constant use in scientific works until the nineteenth century began. Generation is the production of a substance that before was not, and corruption is the destruction of a substance, by its ceasing to be what it was before. Thus, fire is generated, and wood is corrupted, when the latter is burnt. But the implicit metaphor in the use of the terms likens substances to the human body, their production and destruction implies liability to disease, and thus prepares the way for the notion of the elixir, which is first a potion giving long life, and curing bodily ailments, and only after some time a remedy for diseased metals--the philosopher's stone. It will be seen that the theory of the mediaeval alchemist was that matter is an entity filling all space, on which in different places different forms were impressed. The elements were a preliminary grouping of these, and might be present--two, three, or four at a time--in any substance. No attempt was ever made to separate these elements by scientific men, just as no attempt is ever made to isolate the ether of the physical speculations of to-day. The theory of modern physicists, with its ether and vortices, answers almost exactly to the matter and form of the ancients, the nature of the vortices conditioning matter. The extracts from Book XI. bring us to another class of substances. All compound bodies are classified as imperfect or perfect. Imperfect compounds, or meteors, to some extent resemble elements. They are fiery, as the rainbow, or watery, as dew. Our extract on the rainbow is somewhat typical of the faults of ancient science. A note is taken of a rare occurrence--a lunar rainbow; but in describing the common one, an error of the most palpable kind is made. The placing of blue as the middle and green as the lowest colour is obviously wrong, and is inexplicable if we did not know how facts were cut square with theories in old days. In the next extract Bartholomew's account of the spirits animating man is quoted at length. It gives us the mediaeval theory as to the means by which life, motion, and knowledge were shown in the body. Every reader of Shakespeare or Chaucer becomes familiar with the vital, animal, and natural spirits. They were supposed to communicate with all parts of the body by means of the arteries or wosen, "the nimble spirits in their arteries," and the sinews or nerves. The word sinew, by the way, is exactly equal to our word nerve, and ayenward, as our author would say. Hamlet, when he bursts from his friends, explains his vigour by the rush of the spirit into the arteries, which makes