It must be admitted that the bacterial treatment of sewage, though exhibiting such excellent results where it has been given a fair trial, is still in a probationary stage. It appears to stand on reason. The sludge of previous methods is avoided. The sewage is entirely broken down, and the effluent is a comparatively pure one, yet taking back nitrogen, as nitrate, to the soil. The whole change, indeed, in the opinion of Dr. Dupré, is more effective and radical than in chemical treatment. Further, it has been tested as regards its action upon the pathogenic bacilli--those of tubercle and typhoid--with the result that these infective bacteria have been completely destroyed. It appears that such destruction of infective germs occurs in the tank, and depends in degree upon the rapidity with which sewage is passed through the tank. The cultivation beds also have an inimical effect upon infective bacteria. Hence the final effluent is practically germ-free as regards pathogenic organisms. CHAPTER III _BACTERIA IN THE AIR_ METHODS OF EXAMINING AIR FOR BACTERIA The basis of the usual methods in practice is to pass air over or through some nutrient medium. By this means the contained organisms are waylaid, and finding themselves under favourable conditions of pabulum, temperature, and moisture, commence active growth, and thus reveal themselves in characteristic colonies. These are examined, as directed on page 43, by the microscope and sub-culture. Quantitative estimation is not generally made, as a fixed standard is even less a possibility than in milk and soil. Returns of the number of bacteria in the sample taken may be made for the sake of information, but little or no conclusion of value can be drawn from such data. The standard recognised in Europe is the cubic metre, and one may speak, for example, of the air of a room containing 500, 1000, or 3000 germs per cubic metre. The following are the chief methods: 1. _Pouchet's Aëroscope._ This apparatus was in use some time ago in France, and by its means all the solid matter of a given quantity of air was drawn through an air-tight glass tube by aspiration and made to impinge upon a small plate of glycerine. The air escaped to the aspirator at the sides, leaving upon the glycerine plate only its particulate matter. This remnant could then be examined. 2. _Koch_ adopted the simplest of all the culture methods, viz., exposing a plate of gelatine or agar for a longer or shorter time to the air of which examination is desired. By gravity the suspended bacteria fall on the plate and start growth. As a matter of quantitative exactitude, this method is not to be recommended, but it frequently proves an excellent method for qualitative estimation.