3. _The Method of Miquel._ Pasteur was the first to analyse air by the culture method, and he adopted a plan which in principle is _washing_ the air in some fluid culture medium which will retain all the particulate matter, which may then be cultured directly or sub-cultured into any favourable medium. [Illustration: MIQUEL'S FLASK] Miquel has contrived a simple piece of apparatus for the carrying out of this principle. It consists of a flask with a central tube through its own neck for the entrance of the air. On one side of the flask is a tube to be connected with the aspirator, on the other side of the flask a tube through which to pour off the contained fluid at the end of the process. In the flask are placed 30 cc. of sterilised water (or, indeed, if it be preferred, sterilised broth). The entrance tube is now unplugged, and the aspirator draws through a fair sample of the air in the room (say ten litres). This air perforce passes through the water and by the exit tube to the aspirator, and is thereby washed, leaving behind in the water all its bacteria. The aspiration is then stopped, and the entrance tube closed. The water (plus bacteria) is now poured out into test-tubes of media or plated out on Petri's dishes. Provided the apparatus has been absolutely sterilised, and that the water was also sterile, any colonies developing upon the Petri dish are composed of micro-organisms from the air examined. 4. _The Method of Hesse._ This method is somewhat akin to Pouchet's aëroscope, but is in addition a culture method. Hesse's tube is about 2 feet long and 1-1/2 inches bore throughout. At one end is an india-rubber stopper bored for a glass tube to the aspirator. The other end is open. Before using, the tube is sterilised, and 40 or 50 cc. of sterilised gelatine replaced in it. The tube is now rapidly rotated in a groove on a block of ice or under a cold-water tap, and by this simple means the gelatine becomes fixed and forms a layer inside the tube throughout. We have therefore, so to speak, a tube of glass with a tube of gelatine inside it. The apparatus is now ready for use. It is fixed on the tripod, and fifteen litres of air are drawn through, and the tube is properly plugged and incubated at room temperature. In a day or two days the colonies appear upon the gelatine. They are most numerous generally in the first part of the tube, and might be roughly estimated as follows: 15 litres of air, 6 colonies. ⁂ 6/15 × 10,000 = 4000 aërobic bacteria in the cubic metre.