What is protoplasm? In its substance it is a nitrogenous carbon compound, differing only from other similar compounds of the albuminous family of colloid by the extremely complex composition of its atoms. It consists of five elements, and its average composition is said by chemists to be 52·55 per cent. carbon, 21·23 oxygen, 15·17 nitrogen, 6·7 hydrogen, 1·2 sulphur. Its peculiar qualities, therefore, including life, are not the result of any new and peculiar atom added to the known chemical compounds of the same family, but of the manner of grouping and motions of these well-known material elements. It has in a remarkable degree the faculty of absorbing water, so that its molecules seem to float in it in a condition of semi-fluid aggregation, which seems to be necessary for the complex molecular movements which are the cause or accompaniment of life. Thus, many seeds and animalculæ, if perfectly dry, may remain apparently as dead and as unchanging as crystals, for years, or even, as in the case of the mummy wheat, for centuries, to revive into life when moistened. But in addition to those material qualities in which protoplasm seems to differ only from a whole group of similar compounds of the type of glycerine, by the greater complexity and mobility of its molecules, it has developed the new and peculiar element which is called life. Life in its essence is manifested by the faculties of nutrition, sensation, movement, and reproduction. As regards nutrition there is this essential difference between living and non-living matter. The latter, if it feeds and grows at all, does so only by taking on fresh molecules of its own substance on its outer surface, as in the case of a small nucleus-crystal of ice in freezing water. If it feeds on foreign matter and throughout its mass, it does so only in the way of chemical combination, forming a new product. Living matter, on the other hand, feeds internally, and works up foreign substances, by the process we call digestion, into molecules like its own, which it assimilates, rejecting as waste any surplus or foreign matter which it cannot incorporate. It thus grows and decays as assimilation or waste preponderates, remaining always itself. The distinction will be clear if we consider what happens when water rusts iron. In a certain sense the iron may be said to eat the oxygen, reject the hydrogen, and grow, or increase in weight by what it feeds on; but the result is not a bigger piece of iron, but a new substance, rust, or oxide of iron. That living matter should feed internally is not so wonderful, for its semi-fluid condition may well enable foreign molecules to penetrate its mass and come in contact with its own interior molecules; but it is an experience different from anything known in the inorganic world that it should be able to manufacture molecules of protoplasm like its own out of these foreign molecules, and thus grow by assimilation. For instance, when amœbæ, bacteria, and other