As to to the quantitative examination, that is fulfilled by counting the number of colonies which appear, say by the third and fourth day, upon the gelatine plates. Each colony has arisen, it is assumed, from one individual, so that if we count the colonies, though we do not thereby know how many organisms we have upon our plate, we do know approximately how many organisms there were when the plate was first poured out, which are the figures we require, and which can at once be multiplied and returned as so many organisms per cubic centimetre. There is, unfortunately, at present no exact standard to which all bacteriologists may refer. Miquel and Crookshank have suggested standards which allow "very pure water" to contain up to 100 micro-organisms per cc. Pure water must not contain more than 1000, and water containing up to 100,000 bacteria per cc. is contaminated with surface water or sewage. Macé gives the following table: Very pure water 0- 10 bacteria per cc. Very good water 20- 100 " " Good water 100- 200 " " Passable (mediocre) water 200- 500 " " Bad water 500- 1,000 " " Very bad water 1,000-10,000 and over " Koch first laid emphasis on the quantity of bacteria present as an index of pollution, and whilst different authorities have all agreed that there is a necessary quantitative limit, it has been so far impossible to arrive at one settled standard of permissible impurity. Besson adopts the standard suggested by Miquel, and, on the whole, French bacteriologists follow suit. They also agree with him, generally speaking, in not placing much emphasis upon the numerical estimation of bacteria in water. In Germany and England it is the custom to adopt a stricter limit. Koch in 1893 fixed 100 bacteria per cc. as the maximum number of bacteria which should be present in a properly filtered water. Hence the following has been recognised more or less as the standard: 0- 100 bacteria per cc. =a good potable water, 100- 500 " " =a suspicious water. 500-1000 or more " =a water which should have further filtration before being used for drinking purposes.