_The Sowing of Crystals._—Concentrated solutions of various substances, supersaturated solutions of sodium magnesium sulphate, and sodium chlorate are also wonderful culture media for certain mineral organic units—certain crystalline germs. Ch. Dufour, experimenting with water cooled below 0° C., its point of solidification; Ostwald, with salol kept below 39°.5, its point of fusion; Tammann, with betol, which melts at 96°; and, before them, Gernez, with melted phosphorus and sulphur—all these physicists have shown that liquids in superfusion are also media specially appropriate for the culture and propagation of certain kinds of crystalline individuals. Some of these facts have become classic. Lowitz showed in 1785 that a solution of sodium sulphate could be concentrated by evaporation so as to contain more salt than was conformable with the temperature, without, however, depositing the excess. But if a solid fragment, a crystal of salt, is thrown into the liquor, the whole of the excess immediately passes into the state of a crystallized mass. The first crystal has engendered a second similar to itself; the latter has engendered a third, and so on from one to the other. If we compare this phenomenon with that of the rapid multiplication of a species of microbes in a suitable culture medium, no difference will be perceived. Or perhaps we may note one unimportant difference—the rapidity of the propagation of the crystalline germs as opposed to the relative slowness of the generation of the micro-organisms. Again, the propagation of crystallization in a supersaturated or superfused liquid may be delayed by appropriate devices. The crystalline individual gives birth, then, to another individual that conforms to its own type, or even to varieties of that type when such exist. Into the right branch of a U tube filled with sulphur in a state of superfusion Gernez dropped octahedric crystals of sulphur, and into the left branch prismatic crystals. On either side were produced new crystals conforming to the type that had been sown. _Sterilization of Crystalline Media and Living Media._—Ostwald varied these experiments by using salol. He melted the substance by heating it above 39∙5°C.; then, protecting it from crystals of any kind, he let the solution stand in a closed tube. The salol remained liquid indefinitely—until it was touched with a platinum wire that had been in contact with solid salol—_i.e._, until a crystalline germ was introduced. But if the platinum wire has been previously sterilized by passing it, as the bacteriologists do, through a flame, it can then be introduced into the liquor with impunity.