§ V.—New Comparative Studies on the Germs held in Suspension by the Air of Different Places which are near each other, but Subjected to Different Conditions affecting the Production and Diffusion of the Particles of Dust found in them. We may compare the character and the greater or less abundance of similar germs existing in neighbouring localities, by studying the changes which take place in similar liquids exposed simultaneously to the action of the air in those localities. To do this, we must prepare a large number of flasks of the same size, free from air, and containing about equal quantities of a particular liquid—the same being used for all. We must open the same number of these flasks in each of the localities we have selected, and permit the air with all its particles of dust to rush into them; then we must close our flasks again, and observe, day by day, the appearances they present. The results obtained by these means will not furnish us with conclusions applicable to every kind of germ that the air contains at any given moment, but only with conclusions which apply to those germs which can develop in the particular liquid employed. Thus, for example, we could draw no inference as to the nature and relative number of _bacteria_ or _vibrios_, in the case where we employ an acid liquid; for organisms of that kind we must have recourse to neutral or slightly alkaline infusions. On the other hand, liquids having a feeble acid reaction would favour the growth of _mucedines_, _mycoderms_, and certain ferments, as for example, the alcoholic. On November 26th, 1872, we opened and reclosed thirty flasks containing must kept from the last vintage. Ten flasks were opened at the bottom of the garden of the _École Normale_. Ten on the landing of the second floor. Ten in the principal room of our laboratory, which had been swept out shortly before, by which operation the dust of the floor had been raised and put in motion. Different objects made their appearance in a certain number of the thirty flasks on the following days; but from December 17th, things remained stationary. The following observations were made at that date:— Of the ten flasks from the bottom of the garden, only one had undergone any change. Of the ten flasks from the interior of the building, four had undergone change. Of the ten flasks from our laboratory, all had undergone change. The difference in the number of germs held in suspension in the three different places whence we had taken our air was, therefore, considerable. The difference in the nature of the germs was equally marked. Those flasks of our first two series which had undergone change presented no trace of _torulæ_, or anything besides fungoid growths, whilst three of the last ten contained _torulæ_ associated with fungoid growths.[44] [Illustration: Plate 3. Torulæ in Process of Development.]