Some of the simplest living things, consisting of but one microscopic cell, are animals, and some are plants. The essential difference between an animal and a plant is shown very clearly by some of these microscopic creatures. Animals feed on the flesh or “proteid” substances manufactured by other animals or by plants; they also feed on oils or fats, and on the sugar and starch manufactured by other animals or by plants. But they cannot construct these “foods” themselves from the simpler stable chemical compounds called “mineral bodies,” which, nevertheless, contain the elements they require—carbon, nitrogen, hydrogen, and oxygen. Such stable mineral bodies are carbonic acid, ammonia, and water. In fact, ordinary “smelling salts” (which is chemically carbonate of ammonia) dissolved in water, if we add to it a trace of phosphates, sulphates, and chlorides of potash, soda and lime, contain all the actual chemical elements that an animal needs. Yet no animal can be nourished by such a “mineral” soup. On the other hand, it is the special distinction of plants—of green plants, be it noted—that they can feed on this simple diet, and, moreover, cannot feed on anything else. The green colouring matter which gives its beautiful tint to the grass and weeds and the leaves of the big trees which clothe the earth is absolutely essential in this process; so also is sunlight. The living protoplasm of the green-coloured parts of plants is crowded with microscopic discs or plates of a brilliant transparent green colour. The peculiar substance causing the colour is called “leaf-green,” or “chlorophyll.” It can be dissolved out of a leaf, not by water, but by spirit or by ether, and separately studied. It may be seen in solution (to cite a commercial instance) in the liqueur known as “crême de menthe,” being used to give its fine green colour to that preparation. Sunlight shining on to the green parts of plants is “screened” or “strained” by the leaf-green, so that only some of the coloured rays pass through it, and it is only by this peculiarly “strained” green sunlight that the protoplasm of the cells of the leaf is stimulated to its remarkable chemical activity. The carbonic acid in the air or in the water in which the green plant is living is taken up by the protoplasm. Carbonic acid consists of oxygen and of carbon. The protoplasm, when the green sunlight acts on it, actually takes out of carbonic acid and throws off as a gas (seen as bubbles in the case of a water plant) some of its constituent oxygen, thus keeping up the supply of free oxygen in air and water. Then at the same time it combines the carbon and the rest of the oxygen with water (hydrogen and oxygen) inside itself, forming solid starch, which, with the microscope, we can see actually manufactured as little oblong grains in the green cells. Not only this, but the element nitrogen is, so to speak, “forced” in other cells of the plant to combine with the three