ers to suppose that they might be combinations of smaller molecules. Whether these complexes were held together by valence bonds or by mechanical means was an open question.25 During the next four decades, the colloidal state and substances which could enter it received sporadic attention from a variety of biochemists, physical chemists, and physicists. This work resulted in a great deal of empirical data, a proliferation of special terms, and some valuable generalizations. But until the turn of the century, colloids remained an esoteric topic, typically treated in footnotes rather than textbooks. This attitude began to change, however, during the decade or so preceding the World War.

The new interest in this old subject had several sources. In part it owed to advances in technique. New filtration methods, for example, facilitated the isolation of colloidal particles of uniform size; improvements in methods of measuring osmotic pressure and freezing-point depression afforded more reliable means of measuring molecular weights; and with the invention of the ultramicroscope in 1903, it even became possible to watch colloidal particles dancing in solution. Important, too, was the growth of biochemistry and industrial chemistry, which provided an ever-larger number of scientists with opportunity and motive to study colloidal systems. Industry already produced scores of such colloidal products as paints and resins, and was eager to produce others, such as artificial rubber. Many biological substances were also of colloidal dimensions, too small to be observed under an ordinary microscope and too large to diffuse easily across organic membranes. Since Graham's day there had been chemists who suspected that the peculiar properties of living matter might find a basis in the chemistry and physics of colloids. During the years around the turn of the century, a growing number of chemically minded biologists and biochemists, impressed by similari-



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