As M. Miquel remarks, the use of aëroscopes will be of value for the discovery in the air of the germs of the molds which attack our cereals. Regarding the etiology of certain contagious affections, he continues: "It does not seem to be proved that the various spores introduced into our economy, to the number of 300,000 a day, or 100,000,000 a year, are perfectly innocuous. The appearance of thrush in the mouths of young children and in the respiratory canals of the dying seems to demonstrate also that the molds form a part of the class of parasites which are ready to take possession of our organism whenever it presents a vulnerable point or a point of weak resisting power."
M. Pasteur has long insisted on the utility of these researches. "I believe," he wrote in 1862, "that it would be of great advantage to multiply the studies on this subject, and to compare in the same place at different seasons, and in different places at the same time, the number of corpuscles disseminated in the atmosphere. Our knowledge of the phenomena of morbid contagion, especially during the prevalence of epidemics, would, it appears to me, gain from researches prosecuted in this direction."
Since M. Pasteur has established the parasitic character of zymotic diseases like the hen-cholera, sheep-rot, septicæmia, measles, etc., the micrographic statistics of the air has risen to a considerable importance. It has had, however, to concentrate its efforts chiefly upon a class of rudimentary organisms very different from the green algae and the molds of which we have spoken. This group is the one to which the viruses belong. The plants composing it, and which are designated under the common denomination of bacteria, escape the process of numeration in use for the higher cryptogams. In consequence of their extreme minuteness and refractive power, they are
Fig. 4. Specimens of bacteria. A, Micrococcus in isolated cells or aggregated into balls and strings; B, Bacterium; C. Bacillus; a, bâtonnets (adult bacilli; b, bâtonnets with spores; c, isolated spores; d, germinating spores.
invisible, and unrecognizable in the preparations of the aëroscopes. Their existence in the air was long denied, and the proof that they abound in it only dates from the experiments that were instituted by M. Pasteur for the solution of the question of heterogeny. The meth-
