In 1880 Huxley, in association with Professor Roscoe, the chemist, and Professor Balfour Stewart, the physicist, took a great practical step toward securing the widest possible extension of elementary knowledge in science. They became general editors, for the English publishing house of Macmillan, of a series of "Science Primers." These were written in simple language, suitable for those with no preliminary knowledge of science, but were the work of the chief authorities in the leading branches of science. They were published at what was then the phenomenally cheap price of a shilling, and they sold in almost incredible numbers. Huxley himself wrote the introductory volume to this great series of tracts, taking for his subject the simplest and most natural phenomena of the world and the simplest chains of cause and effect that can be observed around us. The keynote of the little book was that knowledge of nature could be gained only by observation and experiment, and that for these the ordinary things in the world around us provided ample material. A few years later he wrote a more advanced volume on the same subject. He had now found an English name for the German _Erdkunde_, and his book on _Physiography_ was simply an account of the leading things and forces of nature. A traveller set down in a foreign land will at once get into difficulties unless he has provided himself with a guide to the geography, the manners and customs, and the regulations of the country in which he finds himself. Huxley's aim was to provide a similar guide to nature; an outline of elementary knowledge of the world into which we all come as strangers. He wrote of force and energy, of the forms of water, of heat and cold, of the atmosphere, of winds and tides and weather, and of the main features of the lives of plants and animals. There was nothing new in what he wrote; he simply took from the chief sciences their leading principles and elementary facts, and set them forth in plain and simple language so that all could read and understand. The novelty was that an attempt should be made to bring these facts within the reach of all. The idea proved extremely infectious; in Europe and America, in many languages and by many authors, Huxley's main lines were followed, with the result that a new branch of education, and almost of science, was created.