_The Dimensions of Crystalline Germs Comparable to those of Microbes._—We may dilute the solid salol with inert powder—lactin, for example—dilute the first mixture with a second, the second with a third, and so on; then, throwing into the solution of surfused salol a tenth of a milligram from one of these various mixtures, we find that the production of crystals will not take place if the fragment thrown in weighs less than a millionth of a milligram, or measures less than ten thousandths of a millimetre in length. It would seem, then, that these are the dimensions of the crystalline particle or crystallographic molecule of salol. In the same way Ostwald satisfied himself that the crystalline germ of hyposulphite of soda weighs about a thousand-millionth of a milligram, and measures a thousandth of a millimetre; that of chlorate of soda weighs a ten-millionth of a milligram. These dimensions are entirely comparable with those of microbes. All these phenomena have been studied with a detail into which it is impossible to enter here, and which clearly shows more and more intimate analogies between the formation of crystals and the generation of micro-organisms. _Extension and Propagation of Crystallization. Optimum Temperature of Incubation._—Crystallization which has commenced around a germ is propagated more or less rapidly, and ends by invading the whole of the liquor. The rapidity of this movement of extension depends upon the conditions of the medium, especially upon its temperature. This is shown very well by Tammann’s experiments with betol. This body, the salicylic ester of naphthol, fuses at 96° C. If it is melted in tubes sealed at a temperature of 100° C., it may be cooled to lower and lower temperatures—to + 70°, to + 25°, to + 10°, to-5° without solidifying. Let us suppose that by some combination of circumstances a few centres of crystallization—that is to say, of crystalline germs—have appeared in the solution. Solidification will extend slowly at the ordinary temperature, at 20° to 25° and thereabouts. On the other hand, it will be propagated with great rapidity if the liquor is kept at about 70°. This point—70°—is the thermal optimum for the propagation of germs. It is the most favourable temperature for what may be called their incubation. As soon as the germs find themselves in a liquor at 70° they increase, multiply, and show that they are in the best conditions for growth.