In another form the apparatus shown in Fig. 45 was used to investigate whether a current of air could take up germs from a liquid at rest. The sewage was placed in a U-shaped tube which formed the trap, and was connected with a flask containing the culture liquid by means of narrow tubing, as shown. This tube passed through rubber stoppers which close the tops of the flask and one arm of the trap. An aspirator tube packed with an asbestos filter was connected with the flask through its rubber stopper, and air was drawn at any desired rapidity through the apparatus, being filtered before entering the trap by an asbestos filter in a small glass tube connecting with the trap through its rubber stopper.

Finally a third form of apparatus, shown in Fig. 46, was used to see if a liquid can part with germs when bubbling, in consequence of the evolution of gases produced by its own fermentation, or of the aspiration of air through its mass. The sewage in this case was held in a small test tube, the filtering tube supplying air descended nearly to the bottom of the sewage so that, as it was drawn through it by the aspirator, it created a bubbling or boiling of the liquid. The aspiration was produced by connecting the aspirator tube with a large flask of water and then drawing off the water through a stop cock connected with the bottom of the flask at any desired speed. The results of these experiments were given in the report as follows:

Fig. 46. Apparatus for determining if a liquid can part with

Fig. 46. Apparatus for determining if a liquid can part with germs when bubbling.

"At normal summer temperatures no germs were given off from the decomposing liquids whenever their surfaces remained unbroken, even though in some of the experiments the air was continuously conducted over them in a slow current. When the surfaces of the liquids were broken, however, by the bursting of bubbles, germs were invariably given off and the sterilized infusions infected, no matter how slowly the aspiration was conducted."

In order to be sure that germs were not conveyed along the inner surface of the bent connecting tube between the trap and the flask containing the infusions, by capillary action, the bend in this tube was kept perfectly free from moisture during the entire time of the experimentation by means of a gentle heat.

"As the tubes," says the report, "were constantly covered with films of moisture from condensed vapors there was no possibility of the bacterial growths drying around the surface edges of the infusions in the inner arms; nor, were that to happen, would it seem probable, judging from Wernich's experiments, that germs could become detached and taken up by the air in the flasks."

The experiments of Naegeli, Wernich, Miquel and others clearly show, also, that under normal conditions germs are not given off to the surrounding air through the evaporation of a liquid containing them, nor from thoroughly moistened sand or solid matter of any kind.