«Solution Theory of the Colloidal Condition.»—For many years the belief was prevalent among chemists that these liquids represent true solutions of difficultly soluble substances, in the form of soluble (colloidal) ‹modifications› of the substances. Like ordinary [p129] solutions, the liquids are found, indeed, to show a certain ‹osmotic pressure›, but, unlike ordinary solutions, the osmotic pressure is exceedingly small in proportion to the quantity of the substance present. Since the osmotic pressure is proportional to the number of molecules in unit volume (Chap. II), this observation proved the presence of a relatively very small number of molecules in solution, and chemists were led to assign, therefore, a relatively very great weight to each. Hence, the data led to the assumption that the colloidal modifications consist of molecules of very large, sometimes enormous molecular weight. From the fact that colloids are unable to traverse membranes, through which crystalloids readily pass, Graham had reached the same conclusion. The following measurements of osmotic pressures may be given:[246] Osmotic Substance. Concentration Pressure. Per cent. Cm. Hg. Gum arabic 1 6.9 Dextrin 1 16.6 As_{2}S_{3} 4 1.7 Fe(OH)_{3} 1.1 0.8 2.0 2.8 3.0 5.6 5.3 12.5 8.9 22.6 The last results, obtained with special care to exclude soluble impurities, are more reliable than the older results on gum arabic and dextrin. For the purpose of comparison it may be said that a 1% solution of cane sugar (molecular weight = 342) has an osmotic pressure of 53 cm. Hg at 18°, a 5% solution a pressure of 265 cm. «The Suspension Theory of the Colloidal Condition.»—On the other hand, a number of chemists considered the colloidal liquids to represent, essentially, ‹suspensions of minute solid particles›[247] in an extreme state of subdivision, or, in some instances, perfect [p130] emulsions of difficultly miscible liquids.[248] Observations with the ultramicroscope[249] finally proved,[250] beyond question, the correctness of this theory of the colloidal condition. Thus, the colloidal "solutions" of gold are seen to contain minute solid particles of gold, the diameter of which varies[251] between (approximately) 60 µµ and 6 µµ and the color of which varies with their size. Still finer subdivisions, the diameter of whose particles cannot be measured, are also found to exist. Colloidal silver, platinum, arsenious sulphide and other colloidal metals and sulphides have also been shown, in the same way, to be suspensions of solid particles.