possible because of man's ingenuity in using the materials offered in the various localities, his habit of taking tools, plants and animals with him and in later periods the maintenance of commerce with the old home.

The daily life of these different groups shows wide differences, and yet underlying all are certain fundamentals which may be indicated. If man is to exist-particularly if he is to acquire any degree of culture — he must secure a permanent food supply, get adequate clothing and find or construct caves, houses or shelters of some sort. In the higher stages some control of light, heat and power is also essential. In addition protection against enemies, whatever these may be, is, to say the least, desirable. The obviously dangerous enemies of primitive man were the large animals, while civilized man must defend himself against the minute forms of life which we may group under the term contagious diseases. Inasmuch as a mere list of man's achievements would fill volumes, we can give here only a few illustrations of the various ways in which man exercises his control. Furthermore, we must limit ourselves to the bases of our own civilization.

Perhaps the most striking evidence of man's control is the decrease in the amount of land required as civilization develops.

"Hunter tribes on the outskirts of the habitable area, as in Arctic America and Siberia, require from 70 to 200 square miles per capita; in arid lands, like the Kalahari Desert and Patagonia, 40 to 200 square miles per capita; in choice districts and combining with the chase some primitive agriculture, as did the Cherokee, Shawnee and Iroquois Indians, the Dyaks of Borneo and the Papuans of New Guinea, 1.2 to 2 square miles per capita.

"Pastoral nomads show a density of from 2 to 5 to the

square mile; practicing some agriculture, as in Kordofan and Sennar districts of eastern Sudan, 10 to 15 to the square mile. Agriculture, undeveloped but combined. with some trade and industry as in equatorial Africa, Borneo and most of the Central American States, supports 5 to 15 to the square mile; practiced with European methods in young or colonial lands, as in Arkansas, Texas, Minnesota, Hawaii, Canada and Argentine, or in European lands with unfavorable climate, up to 25 to the square mile.

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Pure agricultural lands of Central Europe support 100 to the square mile, and those of southern Europe 200; when combining some industry, from 250 to 300. But these figures rise to 500 or more in lowland India and China. Industrial districts of modern Europe, such as England, Belgium, Saxony, Departments Nord and Rhone in France, show a density of 500 to 800 to the square mile." 2

A great change from extensive to intensive agriculture is now taking place in this country. In the Middle West two men will do all the work save at special rush times on a farm of two hundred acres, while in highly developed market gardening in the East about one man per acre is required. Less land is required, but at a steadily diminishing return per man.

In order to carry on his activities man had to be able to use his eyes, that is to see. In the early ages then he had to depend on the light furnished by the sun, moon and stars. Later the glare of the bonfire or the flickering pine knot helped him find his way in the dark. In time the knot yielded place to the torch or lamp with a wick of some vegetable fiber and supplied with animal or vegetable 2 SEMPLE, E. T. o. c., p. 65.

oil. Candles of wax or beef-tallow were made by the Romans and there were guilds of candle-makers in north Europe in the thirteenth century. Sperm candles appear about 1750 and become the standard of light. Even today by the term candle-power we mean a sperm candle of a pound in weight burning 120 grains per hour. A private house was illuminated by coal-gas in 1792 in England; its use in London began in 1807; but it was not common until 1819; while the use of kerosene was not general even in America until after 1859. Electric light was still a curiosity at the Centennial Exposition in 1876, while the regular use of acetylene is still later.

Man's advance in any one art is always conditioned upon the development of other arts whose products must be used in the first. For instance, modern surgery turns upon the use of mirrors, artificial lights, antiseptic dressings and sharp cutting instruments, as well as on a greater knowledge of anatomy. So primitive man learned to construct rude shelters of leaves, branches or tree trunks felled by fire; but, practically speaking, he could neither make nor use lumber until he had possessed himself of metal tools. He came therefore to depend on bricks, sun-dried at first, or on stones for his better structures. When once he had discovered the principle of the arch he could erect massive and enduring monuments. Yet until he had domesticated animals the human cost of a large building must have been enormous. Unless the ancient Egyptians possessed machinery of which all record has been lost, what human energy must have been required in the construction of the pyramids!

We have already seen how important the hydrocarbons are to all organisms. They have been equally valuable to man in his arts. Long ages ago in the Carboniferous

era vast forests grew and died; and by a series of changes, chemical as well as mechanical, were turned into coal in its various forms. We may classify these in accordance with their carbon content, remembering that the series really begins with living wood containing about 50 per cent carbon and ends with the diamond which is practically pure carbon: Lignite, 67 per cent; cannel, 64 per cent; bituminous, 88 per cent, and anthracite, 93 per cent; to which we should add graphite, 95 per cent. That these have different properties and are valuable for various purposes is well known.

Coal was more or less known to the Romans and was slightly used by them as well as by the Chinese. It was mined in small quantities in England before the coming of the Saxons. Little attention was paid to it until the gradual decrease in the forested area by the ninth century. There is an English charter permitting the freemen of Newcastle to mine coal dating from 1239. Thereafter it came gradually into use but at the end of the sixteenth century two ships were able to keep the London market supplied. There was much opposition to its use on the ground of the smoke and dirt and at times it was prohibited particularly during the session of Parliament. In America bituminous coal was mined in Virginia about 1750 and at Pittsburgh in the following decade. Anthracite was discovered at Wilkesbarre in 1762 but was little used, as the people did not know how to burn it. It was used at Carlisle during the Revolution for the manufacture of firearms but as late as 1820 the Philadelphia market was satisfied with 365 tons from Lehigh.

The first coal product to be commercially used was illuminating gas which dates, as above indicated, from the end of the eighteenth century. In the process of manu

facture many other substances are given off which were wasted at first but which have now become very valuable. The first of these is crude tar; then by further distillation, naphtha, ammonia, benzene, vaseline, the heavy lubricating oils, paraffin, aniline and others. By different processes we get coke, soot and lampblack.

Natural petroleum was known at least five centuries before Christ and was used, it is believed, in keeping alight the sacred fires in the Persian temples. Marco Polo writing about A.D. 1260 mentions its use. Until the discov

ery of the oil wells of Pennsylvania it had little importance. So too asphalt seems to have been known and used in Egypt and Babylon but only recently, through the discoveries of the lakes in Trinidad and elsewhere, has it come into general use.

Now either directly from coal or from related substances come a considerable percentage of some of our most highly valued commodities. Aniline dyes which have made possible the colors of our clothing materials were developed after 1836 and became important by the middle of the century. Another product is carbolic acid.

Meantime coal, kerosene and gasolene furnish us with light for our houses and streets, with fuel for our engines and motors, with heat for our buildings; and have made possible a wonderful change in our daily life and industrial processes. Through their use we are able to make iron the basic material of our economic life.

The use of the sharp edge of shells or slices of stone as scrapers or knives marked a great advance. A new era dawned when man first discovered the softer metals, copper, lead and tin. The next stage is marked by the making of the alloys brass and bronze. Relatively late, man began to forge tools of iron and really entered upon the