PROCEEDINGS Ol" THE COTTESWOLD CLUB I27 



causes a liquificution of the nutrient gelatine preparation, 

 the presence of the " colony " being indicated by a liqui- 

 fying spot in the gelatine. The nutrient preparation has 

 sufficient gelatine in it to form a solid jelly at temperatures 

 below lOO'^F. and the modus operandi is to melt a suffi- 

 cient quantity in a test tube, add the measured drop of 

 water, and before the mixture sets pour it out in an equal 

 layer into a covered glass plate. In the course of two or 

 three days, the time varying according to the temperature, 

 the liquifying spots in these plate cultivations begin to 

 ap[)ear, each spot represents one bacterium in the added 

 drop of water ; the spots grow in size rapidly and soon 

 the whole of the gelatine is involved and has become 

 entirely liquified, but before this has occurred, whilst the 

 spots are still distinct, they are counted and a portion of 

 nutrient gelatine in a test tube may be innoculated from 

 any one of the spots, by touching the spot with the point 

 of a needle and conveying what adheres to the needle to 

 the gelatine in the tube, this being afterwards kept free of 

 other microbes by plugging with cotton wool, which is 

 found to act as a perfect filter for the bacteria of the air, 

 allowing none to pass it. In this way a crop of the in- 

 dividual species of bacteria is prepared, though sometimes 

 it may be necessary to make several successive plate 

 cultivations in order to get an undoubted separation of 

 species. DiflFerent species of bacteria behave in different 

 ways towards various cultivating media, such as milk, 

 cooked potatoes, and the like, many of them in their culti- 

 vations also exhibit a colour and mode of growth peculiar 

 to themselves, and such differences together with slight 

 differences in size and shape enable many of them to be 

 distinguished. 



Bacteria exist in all exposed waters in amazing numbers. 

 They are too numerous to be mentioned in connection 

 with the gallon or the pint, but one cubic centimetre, which 

 is about one thirtieth part of an ounce, is the measure of 



