to raise the temperature of the body 57°. If the body were built of most substances, it would raise the temperature 180° to 200°. Man loses, therefore, about 1.5 calories per kilo per hour. Some of the elements in the body, moreover, would undergo marked changes if the temperature were much higher than it is. Our clothing does not give heat to the body, and is a better conductor of heat than air. It is chiefly of value because it keeps a thin sheet of air next to the body.

Of all known substances, water is best fitted to maintain an even temperature. Its specific heat is very high, being exceeded only by hydrogen and ammonia. If atomic weight is considered in addition, it stands at the top of the list. The freezing point of water is perhaps 100° higher than that of the average substance. This means that the latent heat of water is very high, being surpassed only by ammonia. Thanks to the expansion that occurs when ice forms, the actual freezing of more than the surface of a body of water is practically impossible. Water evaporates readily. It is estimated that at the equator some six and a half feet of water are taken yearly from the surface of the ocean. This spreads over the earth cooling the tropics and warming other regions. In these regards the only rival again is ammonia, but nowhere in nature is this found in large amounts. Edison states that the greatest discovery of the year 1913 was that of the method of producing ammonia directly from hydrogen and carbon. Water is a splendid solvent. It thus provides opportunity for many of the combinations necessary to life. Its surface tension (75), higher than any other substance save mercury (436), is very significant in view of capillary action enabling it to penetrate

the earth and the cells of organisms. Altogether, therefore, water seems to possess unique qualifications.

Another element of utmost importance to living organisms is carbon although this forms only .03 per cent by volume of the atmosphere and .01 per cent by weight of water. It is stated that over 100,000 compounds of carbon are now known. Some of these are very simple like carbonic acid (CO2); others are so complex that a line would be required to write the formula. This element is peculiar in that it has the remarkable power of entering into union with itself as well as with other substances and is thus able to build up large molecules containing a great number of atoms which form the basis of living structures. The complex carbon molecules are relatively stable and thus give a needed permanency to delicate organic substances. "It is a curious fact in nature that there seems to be a position of greater stability when groups of six carbon atoms unite in little galaxies or concatenations. . . . . Although such groups of less, or occasionally more, than six carbon atoms are formed naturally by life processes, by far the greater portion of the substance of living organisms is built up of six carbon groups and multiples of these." 1

Carbonic acid enters and leaves water freely and is always associated with it. From the first " it has steadily fulfilled the function of regulating the reaction of protoplasm and of body tissue and fluids. The one chemical process which is open, if any transformations whatsoever are to be accomplished with carbonic acid and water, leads directly and to all appearances necessarily to the greatest complexity that has been found in any one chemical pro1 MOORE, B. Origin and Nature of Life, p. 105.

2

cess to a system made up of possibly two hundred substances or more, most of which possess very great chemical activity." The "unparalleled instability" of hydrogen and oxygen with the stability of some of the other factors makes possible the wonderful and delicate adjustment of the body between the conflicting demands of permanency and change.

The three great classes of food are the carbohydrates, the sugars and starches, which contain from thirty to forty of these great carbon groups joined together; the fats, formed by the substitution of hydrogen for part of the oxygen in the carbohydrates; and the proteins, which differ from carbohydrates and fats in that they contain nitrogen and sometimes phosphorus, sulphur and iron. Substances not in the food group or taken in excessive amounts must be eliminated. Here again carbon and oxygen play a great part. The lungs are stimulated into motion by carbonic acid. The oxygen enters and is carried through the system. The objectionable substances are broken down and burned to final products, usually carbon and water. We are not surprised to learn then that a man weighing 60-70 kilograms excretes daily: water, 2500-3500 grams; carbon dioxid, 750-900 grams; all other substances, 60-125 grams. Thus, in that wonderful process of building up and tearing down called metabolism, we gain the desirable elements and eliminate the harmful. How these elements are changed into living structure we know not the fact has become clear. We are all familiar with the old illustration of the lily beautiful in spite of the mud in which it grows. We see that the lily is beautiful because of the mud. Did not these

2 HENDERSON, L. J. The Fitness of the Environment, p. 219.

inorganic substances possess their peculiar properties, life could not be.

It was not enough for nature to furnish the basic elements of life. These had to be gathered together and made available. This leads us to the study of soils. A few plants like some mosses can live directly on rocks drawing their sustenance chiefly from the air. Opportunity for root growth is essential to the higher types. Soils have been produced by the disintegration of rocks and the decay of vegetation age after age in the place where the soil now is, or else they have been formed by the action of rain, wind and ice. We may thus classify the chief sorts:

Soils of these various types will have different characteristics. The soil particles will vary in size from clay, so fine that it can scarcely be measured, to coarse gravel. They will vary in weight per cubic foot, exclusive of water, from peat at 40 pounds; clay, 75; half sand and half clay, 96; to siliceous sand, 110. The texture of the soil is fixed by the arrangement of its particles and this determines the moisture it may hold. In fine clay the pore space is about 50 per cent but in coarse sand only 25 per cent. Evaporation takes a large amount of water from the soil, hence a covering of fine particles helps to retain the moisture. For one ton of clover hay some 1560 tons of water are used. Here again the significance of water is indicated. Not merely does it furnish a large

part of the substance of the plants, but through the affinities of hydrogen and oxygen it combines with the various soil elements and makes them available as plant food. Once in a while the results are less happy. In countries of good rainfall the soluble salts of calcium, magnesium, sodium and potassium are leached out of the soil and carried off. In arid regions they are often brought to the surface and deposited as alkali which seriously checks and even prevents plant growth.

Inasmuch as plants must have certain chemicals it is evident that the fertility of the soil depends in part upon the amount of these present. If one essential is lacking, growth is checked, no matter how abundant the other elements.

In thirty-four soils analyzed by American chemists, the first eight inches of soil of an acre contained, on an average, potential plant food as follows: nitrogen, 3,217 pounds; phosphoric acid, 3,936 pounds and potash, 17,597 pounds; a total of 24,750 pounds or more than 12 tons.

Since these elements are taken out of the soil by plants year by year they must be returned in some way or else the soil will grow sterile. In good measure this is accomplished by the decay of the plants. The fact that the value of complete fertilizers manufactured in the United States increased from $26,318,000 in 1900 to $31,305,000 in 1905, or 18 per cent, indicates man's recognition of the necessity of replacing these food elements. Plant life soon shows any decrease in needed supplies by spindling growth, poor color and little fruit.

Different plants make different demands upon the soil. When the grains languish, while sorrel and oxeye daisy thrive, it is found that nitrogen is lacking. Recently we learned that the essential but not abundant element