One or two thousand cubic centimetres of the water under examination are passed through a sterilised Berkefeld filter by means of siphon action or an air-pump. The candle of the filter retains on its outer surface all, or nearly all, the particulate matter contained in the water. The matter thus retained on this outer surface is brushed by means of a sterile brush into 10 or 20 cc. of sterilised water. Thus we have all the organisms contained in two litres of the water reduced into 10 cc. of water. From this, so to speak, concentrated emulsion of the bacteria of the original water, phenol-gelatine plates or Eisner plates (both acid media) may be readily made. In this way we not only catch many bacteria which would evade us if we were content with the examination merely of a few drops of the water, but we eliminate by means of the acid those common water bacteria, like _Bacillus fluorescens liquefaciens_, which so greatly confuse the issue. [Illustration: HOT AIR STERILISER For the Sterilizatin of Glass Apparatus, etc.] In the course of two or three days the film of gelatine on the plate becomes covered with _colonies_ of germs, and the next step is to examine these quantitatively and qualitatively. We may here insert a simple scheme by which this may be most fully and easily accomplished:-- 1. _Naked-Eye Observation of the Colonies._ By this means at the very outset certain facts may be obtained, viz., the size, elevation, configuration, margin, colour, grouping, number, and kinds of colonies, all of which facts are of importance, and assist in final diagnosis. Moreover, in the case of gelatine plates (it is otherwise in agar) one is able to observe whether or not there is present what is termed _liquefaction of the gelatine_. Some organisms produce in their development a peptonizing ferment which breaks down gelatine into a fluid condition. Many have not this power, and hence the characteristic is used as a diagnostic feature. 2. _Microscopic Examination of Colonies_, which confirms or corrects that which has been observed by the naked eye. Fortunately some micro-organisms when growing in colonies produce cultivation features which are peculiar to themselves (especially is this so when growing in test-tube cultures), and in the early stages of such growths a low power of the microscope or magnifying glass facilitates observation. 3. _Make cover-glass preparations_: (_a_) unstained--"the hanging drop"; (_b_) stained--single stains, like gentian-violet, methyl blue, fuchsin, carbol fuchsin, etc.; double stains--Gram's method, Ziehl-Neelsen's method, etc. [Illustration: THE HANGING DROP]