nearly every tube that receives a centimetre of the water will become turbid; but, if the number of bacteria is only half as many, half the tubes will remain sterile. This rule, though inexact in theory, seems to prevail with an approach to exactness in practice. M. Miquel applied it to the estimation of the bacteria in rain-water, and found that at the beginning of storms the water of precipitation contained a considerable number, amounting sometimes to as many as fifteen per cubic centimetre, and that the number immediately began to diminish; but that, strange to say, "after two or three days of moist and rainy weather the meteoric water frequently contained more bacteria than at the beginning of the rain. As the atmosphere was then in a condition of extreme purity—a fact that was established simultaneously by the statistics of the germs in the air—it seemed to be shown that the bacteria could live and multiply in the very midst of the clouds, or, perhaps, that the clouds might in their course through space charge themselves with a very valuable contingent of germs."
In studying under the microscope the development of these little organisms, in the preparations of which they have taken possession, a very curious evolution of one of the microbes of the air is revealed. The organism is a bacterium, which presents at first sight the characteristics of a very long, filamentous bacillus. M. Miquel affirms that he has seen this organism afterward divide itself into segments of unequal size, in such a way as to form strings of micrococci. The observation deserves consideration, for, if it is confirmed, and the
Fig. 7.—Successive Phases in the Transformation of the same Organism (after M. Miguel). Magnified 1,000 diameters.
habit is proved to be general, it will establish a line of union between the different types of the inferior algae, which were believed to be fixed, but may be, after all, only transient genera.
It is important to have the microbes collected in the broth of the tubes sown in different kinds of liquors. Such treatment furnishes
