Some observations in the preceding chapter enable us to account for the vast number of germs which are constantly falling on the surface of everything. We may here allude to the use we have made of flasks, shaped as in Fig. 17, and holding from 250 c.c. to 300 c.c., which are a third part filled with an organic liquid, and are closed up when boiling. They contain no air when cool, and are opened in series of 10, 20, &c., out of doors, and closed up again immediately. The air rushes violently into the vacuum, and thus we introduce about 200 c.c. of air, with all the particles of dust contained in that air, into each flask. It has been proved that a certain number of these flasks undergo change in the course of time, the number of those changing and the nature of their changes being in close proportion to the probable number and nature of the floating germs able to develop in the particular nutritive liquid used. If we work at great elevations, far from houses and the dirt of towns and inhabited plains, as we did at Montanvert, near the _Mer de Glace_, change will seldom occur. The opposite will be the case if we work in a place like the living-room of the little, dirty, ill-kept inn at Montanvert. In a laboratory where fermentation is studied we obtain certain kinds of germs which often differ from those found in the air of the open country. If we desire to have organisms in all our flasks, we have only to stir up the dust on the ground or on surrounding objects at the moment when we open the flasks. This simple and easy experiment clearly shows us that it is impossible for a field of sporanges of fungoid growth, existing in an uncovered vessel or on the surface of a fruit, to escape becoming mixed with germs that are foreign to the little plant; in other words, the student who sows spores of _penicillium_, which he has collected from one place or another on a brush, exposes himself to serious causes of error. Footnote 52: M. Jules Raulin has published a well-known and remarkable work on the discovery of the mineral medium best adapted by its composition to the life of certain ordinary fungoid growths; he has given a formula for the composition of such a medium. It is this that we call here “Raulin’s fluid” for abbreviation. Water 1,500 Sugar Candy 70 Tartaric Acid 4 Nitrate of Ammonia 4 Phosphate of Ammonia 0·6 Carbonate of Potassium 0·6 Carbonate of Magnesia 0·4 Sulphate of Ammonia 0·25 Sulphate of Zinc 0·07 Sulphate of Iron 0·07 Silicate of Potassium 0·07 J. RAULIN. Paris, Victor Masson, 1870. _Thèse pour le doctorat._ Footnote 53: