A Chemical Sign of Life

ctivity, and that the nerve fiber is no exception to this rule. In other words, respiration is one of the unfailing signs of life and is a necessary condition for living processes. But carbon dioxide/production from a tissue is not by itself a sufficient sign of life. For there are many chemical compounds which spontaneously give off carbon dioxide, among others sea-water, bicarbonate solutions, as well as organic materials which are unstable. It would obviously be a mistake to call these compounds living because of the fact that they give off this gas. This criterion alone, therefore, cannot be used for detecting the vitality of the tissues. Not only is it common for many non-living matters to give off carbon dioxide spontaneously, but there are also some whose mode of gaseous exchange is remarkably similar to that of the living process. Among these substances there is none in which the parallelism to vital respiration is more detailed and interesting than ordinary linseed oil. The many curious resemblances of the chemical processes involved in painting to proto- plasmic respiration and growth have already been pointed out.1 It is unnecessary to go into this more fully than to call attention to the facts that linseed oil takes up oxygen, that it gives off carbon dioxide, that it is stimulated by light, that it undergoes also other phases of metabolism common to living matter, and that, very singularly, it exhibits many of the phenomena of memory, learning, ...