But the same happens when we hinder the passing out of the products of chemical transformation from the cell. On the other hand, we may keep a crystal of lifeless matter in a glass tube carefully shut up from all exchange of substance with the external world for as many years as we like. The existence of this crystal will continue without end and without change of any of its properties. There is no known living organism which could remain in a dry resting state for an infinitely long period of time. The longest lived are perhaps the spores of mosses which can exist in a dry state more than a hundred years. As a rule the seeds of higher plants show their vital power already weakened after ten years; most of them do not germinate if kept more than twenty to thirty years. These experiences lead to the opinion that even dry seeds and spores of lower plants in their period of rest of vegetation continue the processes of metabolism to a certain degree. This supposition is confirmed by the fact that a very slight respiration and production of carbonic acid can be proved when the seeds contain a small percentage of water. It seems as if life were weakened in these plant organs to a quite imperceptible degree, but never, not even temporarily, really suspended. "Life is, therefore, quite inseparable from chemical reactions, and on the whole what we call life is nothing else but a complex of innumerable chemical reactions in the living substance which we call protoplasm."[1] The essential quality, therefore, of life is continual change, but not mere change in general. It is that special process of the circulation of matter which we call metabolism, and this circulation is always connected with a particular chemical substance--protoplasm. In this substance five elements are always present and predominant--carbon, oxygen, nitrogen, hydrogen, and sulphur. The compounds which these five elements form with each other are most complex and varied, and they also admit to combination--but in smaller proportions--some of the other elements, of which phosphorus, potassium, calcium, magnesium, and iron are the most important. For protoplasm--using the term in the most general sense--is a chemical substance, not a mere mixture of a number of chemical elements, nor a mere mechanical structure. "However differently the various plasma substances behave in detail, they always exhibit the same general composition as the other albuminoids out of the five 'organo-genetic elements'--namely in point of weight, 51-54% carbon, 21-23% oxygen, 15-17% nitrogen, 6-7% hydrogen, and 1-2% sulphur."[2]