Scheel discovered oxygen. By the end of the century . Cavendish had resolved water into its component parts. A few years later Davy, experimenting with his electric battery, discovered that all the common substances known as "earths" were compounds containing oxygen. Thenceforth there was a steady series of discoveries of elements whose qualities were noted and whose atomic weights it proved possible to determine. In 1868, by using the spectroscope, Johannsen of Paris found in the sun an unknown substance which he called helium and in 1895 Ramsey was able to show its presence on earth. The most striking of these discoveries was probably that of radium in 1898 by Madame Curie.

There are listed today some eighty-three elements which, so far as we can tell, are distinct. Nearly 99 per cent of the earth's surface including water (as the following table shows) is made up of eight of these, no other element contributing one per cent. Even such extremely important substances as hydrogen and carbon form only .17 per cent and .12 per cent, respectively.

Some of the elements, like gold, are very passive and are seldom found in combination with others. On the other hand, some 60 per cent of known inorganic compounds contain oxygen; some 30 per cent, hydrogen. Many of these compounds possess very different properties from those held by their constituent elements, and

may themselves enter as units into more complex groupings. Some of the elements have not been found in nature save as compounds and man has reduced them in his laboratory. Aluminum, first produced in 1827, has become commercially important; while the metal calcium remains merely a curiosity. In like manner many possible compounds, not found on earth, have been made by the chemist. These elements existing throughout the universe, wherever found so far as we know, possess the same qualities and are governed by the same laws. Thus the earth is revealed to us as made out of the same world stuff" as the planets and the suns. We know too that these elements (depending on temperature and pressure) assume solid, liquid or gaseous forms.

The discovery of the radio-active substances suggests that there has been an evolution of the elements and the belief is growing that they are perhaps but different forms of some common basic material. Radium appears to have been derived from uranium and seems itself to change into helium, and there is suspicion that the final product is lead. The atomic weight of the elements can be arranged in a complex series in a way that indicates that there must be some relationship. In some instances the absence of an element filling a given niche in the series has resulted in its discovery. We are beginning to speak of "electrons " and "ions" and to realize that matter is not the inert thing we have thought it to be. Radium gives off three classes of rays. The alpha rays seem to be minute bodies, positively electrified, which travel some 20,000 miles a second and may be stopped by a sheet of aluminum 1/500 of an inch in thickness. The beta rays are negatively electrified, travel 10,000 miles a second, and are stopped by a sheet of aluminum 1/25 of an

inch thick; while the gamma rays, apparently the same as X-rays, will pierce a plate of aluminum up to twenty inches or take a photograph through a foot of iron. Such facts are compelling us to wonder if force as well as substance is inherent in the concept of matter, and to realize that our chemical formulae do not fully describe the various substances.

With the growth of knowledge with reference to matter the physicist has discovered that those elements or compounds which enter largely into the structure of living organisms are not merely those whose properties enable them to maintain life but are the only ones possessing these properties. He emphasizes, therefore, the reciprocal nature of the relationship between the physical environment and the organism. Before considering this "" fitness of the environment we must note some of the characteristics of living organisms.

In earlier days life was considered as something entirely apart from matter, an outside spirit temporarily living in a material body; just as light and heat were interpreted as occult manifestations of some spirit. A great gulf existed at that time between organic and inorganic chemistry. In 1828 Woehler succeeded in producing urea from ammonium cyanate; in other words, synthetically created an organic compound up to that time known only as the product of living organisms. Today the chemist can create dozens of such compounds and the gulf between organic and inorganic is bridged even though we do not understand life itself. Body as well as spirit is inherent in the concept of life on earth and the existence and phenomena of living beings depend on the properties of matter, rather than on so-called "vital principles."

It is necessary then to think of the organism as an engine, made up of a multitude of parts, each having its own functions and the welfare of the whole depending on the proper functioning of each part. The foodstuffs must be transformed by a process of combustion (which we call digestion) into flesh, bone and energy. In this process there is the most wonderful coöperation between the different parts of the body. Even some very small organs, such as the ductless glands, until recently considered as of little importance, are now known to be indispensable. Thus the suprarenals, two glands not over one ounce in weight, situated above the kidneys, secrete adrenalin, a powerful constrictive having great influence on the arteries. Cretinism is produced by the nonfunctioning of the thyroids. Hypertrophy of the pituitary leads to giantism. The welfare of the whole body depends upon the performance of its own task by each portion thereof. That this is possible is due to the properties of certain compounds soon to be considered. We are able to see because the lens of the eye focuses the rays of light upon the retinal red and takes a picture, just as does the camera, which in some mysterious and wonderful fashion is recorded and filed in the brain. Life then may be conceived of as a series of chemical changes. In order that there may be permanency of the body there must be certain relatively unchanging portions; in order that new experiences may be had there must be instant and appropriate response to new conditions and stimuli.

On the earth today wherever and whenever conditions are favorable, there is life. The lowest forms are found either in water or under moist surroundings. Where the air is dryest the least life exists. If this has always

been so we must assume that when the temperature of water on the cooling earth dropped below the boiling point, 212°, the lowest forms appeared. The hydrocarbon group of compounds is then the first to attract our attention.

Pure air is composed of 79 parts nitrogen, 21 of oxygen and about .03 per cent of carbon dioxid with insignificant traces of argon, metargon, krypton and neon. Pure water is made of two parts hydrogen to one of oxygen. The human body contains carbon, 13.5 per cent; hydrogen, 9.1 per cent; oxygen, 72 per cent; nitrogen, 2.5 per cent; phosphorus, 1.15 per cent; sodium, 0.1 per cent; calcium, 1.3 per cent; magnesium, 0.001 per cent; iron, 0.01 per cent and traces of silicon and fluorin. Plants are composed largely of oxygen, hydrogen and carbon.

We may consider the maintenance of a fairly fixed temperature as one of the first conditions of life. Some of the bacteria 'live and multiply in water at a temperature of 32°, others prosper at 170°. Plants have their standard temperature which falls some 2 or 3° at night. The normal temperature of the human body is slightly above 98°, being lower at night and lowest in early morning hours. Muscular exertion raises the temperature about one degree. If the temperature rises to 105° we have a dangerous fever; if it is depressed to 93° the result is probably death. We perish from cold long before the body is actually frozen. To maintain this temperature a large part of the fats and carbohydrates in the food is burned (oxidized) within the body. Here the importance of water appears. By weight some 68 per cent of the body is water. An adult at rest weighing 165 pounds produces some 2400 calories (heat units) daily, enough