Compounds and Cells. Gases and Life

While species were being successfully classified, the science of life was being extended in a new and extremely fruitful direction. The study of chemistry was being revolutionized and chemists began to apply their techniques to living organisms as well as to inanimate systems. That this was a legitimate thing to do was clearly demonstrated in one early experiment on digestion.

Digestion is one function of the animal body that is relatively open to investigation. It does not take place within the body tissues themselves, but in the food canal which is open to the outside world and can be reached by way of the mouth. In the seventeenth century there had been a serious question as to whether digestion was a physical process involving the grinding action of the stomach, as suggested by Borelli (see page 26), or a chemical process involving the fermenting action of stomach juices, as suggested by Sylvius (see page 27).

A French physicist, Rene Antoine Ferchault de Reaumur (1683-1757), thought of a way of testing this. In 1752, he placed meat in small metal cylinders open at both ends (the ends being covered by wire gauze) and persuaded a hawk to swallow them. The metal cylinder protected the meat from any grinding action, while the wire gauze permitted stomach juices to enter, without allowing the meat to fall out. Hawks generally regurgitate indigestible matter and when Reaumur's hawk regurgitated the cylinder, the meat inside was found to be partially dissolved.

Reaumur double-checked by having the hawk swallow and regurgitate a sponge. The stomach juices that saturated the sponge were then squeezed out and mixed with meat. The meat slowly dissolved, and the issue was settled. Digestion was a chemical process and the role of chemistry in life was effectively dramatized.

In the eighteenth century, the study of gases, begun by Van Helmont (see page 27), was progressing with particular rapidity and becoming a glamorous field of study. It was inevitable that the connection of various gases with life be explored. An English botanist and chemist, Stephen Hales (1677-1761), was one of the explorers. He published a book in 1727, in which he described experiments by which he measured the rates of plant growth, and the pressure of sap, so that he is considered the founder of plant physiology. He also, however, experimented with a variety of gases and was the first to recognize that one of them, carbon dioxide, contributed somehow to the nourishment of plants. In this he corrected (or, rather, extended) Van Helmont's view that it was water alone out of which plant tissues were formed.

The next step was taken by the English chemist, Joseph Priestley (1733-1804) a half-century later. In 1774, he discovered the gas we now call oxygen. He found that it was pleasant to breathe and that mice were particularly frisky when placed in a bell jar containing oxygen. He further recognized the fact that plants increased the quantity of oxygen in the air. A Dutch physician, Jan Ingenhousz (1730-99), showed, moreover, that the process by which plants consumed carbon dioxide and produced oxygen took place only in the presence of light.

The greatest chemist of the age was the Frenchman, Antoine Laurent Lavoisier (1743-94). He emphasized the importance of accurate measurement in chemistry and used it to develop a theory of combustion that has been accepted as true ever since. According to this theory, combustion is the result of a chemical union of the burning material with the oxygen of the air. He showed also that, in addition to oxygen, air contains nitrogen, a gas that does not support combustion.

Lavoisier's "new chemistry" had its applications to life forms, too, for in some ways what applied to a candle applied to a mouse as well. When a candle is set to burning in a closed bell jar, oxygen is consumed and carbon dioxide is produced. The latter comes about through the combination of the carbon contained in the substance of the candle with the oxygen. When all or almost all the oxygen in the air within the bell jar is consumed, the candle goes out and will no longer burn.

The situation is similar for animal life. A mouse under a bell jar consumes oxygen and forms carbon dioxide; the latter through the combination of the carbon in its tissue substance with oxygen. As the oxygen level in the air drops, the mouse suffocates and dies. From the over-all point of view, plants consume carbon dioxide and produce oxygen, and animals consume oxygen and produce carbon dioxide. Plants and animals together, then, help maintain the chemical balance so that, in the long run, the atmospheric content of oxygen (21 per cent) and of carbon dioxide (0.03 per cent) remain steady.

Since a candle and an animal both produce carbon dioxide and consume oxygen, it seemed reasonable to Lavoisier to suppose that respiration was a form of combustion and that when a particular amount of oxygen was consumed, a corresponding quantity of heat was produced whether it was a candle or a mouse that was involved. His experiments in this direction were necessarily crude (considering the measuring techniques then available) and his results only approximate, but they seemed to bear out his contention.

This was a powerful stroke on the side of the mechanistic view of life, for it seemed to imply that the same chemical process was taking place in both living and nonliving matter. This made it that much more reasonable to suppose that the same laws of nature governed both realms as the mechanists insisted.

Lavoisier's point was strengthened as the science of physics developed during the first half of the nineteenth century. In those decades, heat was being investigated by a number of scientists whose interest was aroused by the growing importance of the steam engine. Heat, by means of the steam engine, could be made to do work, and so could other phenomena, such as falling bodies, flowing water, air in motion, light, electricity, magnetism, and so on. In 1807, the English physician, Thomas Young (1773-1829), suggested "energy" as a word to represent all phenomena out of which work could be obtained. It comes from Greek words meaning "work within."

The physicists of the early nineteenth century studied the manner in which one form of energy could be converted to another, and made increasingly refined measurements of such changes. By the 1840s, at least three men, an Englishman, James Prescott Joule (1818-89), and two Gemians, Julius Robert von Mayer (1814-78) and Hermann Ludwig Ferdinand von Helmholtz (1821-94), had advanced the concept of the "conservation of energy." According to this concept, one form of energy might be freely converted into another, but the total amount of energy could neither be decreased nor increased in the process.

It seemed natural for such a broadly general law, based on a wide variety of meticulous measurements, to apply to living processes as well as nonliving. The mere fact that no living animal could continue living without obtaining energy continuously from its food made it seem that life processes could not create energy out of nothing. Plants did not eat and breathe in quite the same way animals did, but, on the other hand, they could not live unless they were periodically bathed in the energy of light.

Mayer, indeed, specifically stated that the source of all the various forms of energy on earth was the radiation of light and heat from the sun; and that this was likewise the source of the energy that powered living organisms. It was the direct energy source for plants and, through plants, for animals (including, of course, man).

The suspicion grew, then (and was to be amply demonstrated in the second half of the nineteenth century), that the law of conservation of energy applied as strictly to animate nature as to inanimate nature and that in this very important respect, life was mechanistic.