Linseed-oil contains large quantities of linolein, which is the substance that oxidizes, forming the stiff, rubbery coat on the surface of linseed-oil when exposed to the air. This makes linseed-oil valuable matter to the painter, but objectionable as a food. LESSON IX DRUGS, STIMULANTS, AND NARCOTICS With the origin and the use of drugs in the treatment of dis-ease, most people are familiar. The purpose of this lesson, however, is to give brief but accurate information concerning the various chemical elements and compounds termed drugs or medicines. Many of the medicines in common use are neutral, having no particular effect upon the body, and the effects attributed to them are largely imaginary. Out of the many thousands of chemical materials found in nature, there are, however, certain substances, groups, and compounds which have most marked and violent effect upon all forms of living protoplasm. [Sidenote: Ancient belief concerning medicine] The general theory upon which the practise of medicine rests is that certain chemical substances which are not found in the animal body, and which have no natural place therein, have mysterious and beneficial effects; that they possess certain powers, among which are the rebuilding of dis-eased cells, and the purifying of dis-eased blood. This belief arose in a very remote age, when the mind was primitive; when man was ignorant, and controlled almost wholly by superstition--when every natural phenomenon was believed to be the work or whim of some god, and every dis-ease was thought to be the work of some devil. [Sidenote: Life the result of chemical harmony] Modern science has proved all this to be untrue. We know by the selective processes through millions of years of evolution that those chemical substances which work in harmony have become associated so as to form life. We know that life is merely an assemblement of organic matter, very complex and little understood; that it is eternally undergoing chemical changes governed by the natural laws of development and decay. We know that conformity to certain natural laws will produce physical ease, and that violation of these laws will produce dis-ease. We know that ease is what we most desire, therefore the trend of thought, throughout the world, is to realize this desire by turning toward the natural. [Sidenote: The material upon which life depends] True food furnishes the foundation or constructive material upon which all life depends. Nearly all other substances which affect the human body are merely disturbing elements that interfere with the natural chemical processes of life.