596 Transactions of the Society. 



organisms the Microscope is of little use ; recourse must be had to 

 the study of their life-history, more especially of their peculiarities 

 of growth on different soils. Of all the materials employed as 

 cultivating media, Koch's gelatinized meat infusion is the most 

 useful for purposes of diagnosis. This is composed of an infusion 

 of meat containing 1 to 3 per cent, of pepton, 1 per cent, gelatin 

 made neutral by carbonate of soda, and thoroughly sterilized. This 

 material was tirst introduced with the view of having a highly 

 nutritive solid and at the same time transparent medium, on which 

 to carry on pure cultivations, but it was soon found that owing to 

 the remarkably diverse ways in which different micro-organisms 

 grew in it, it could be used as a means of diagnosis of the kind of 

 organism, a means more certain than any other which we at 

 present possess. For purposes of diagnosis as well as with the 

 view of carrying on pure cultivations this material is used in three 

 ways. While the material is still fluid a small portion is poured 

 into a number of pure tubes plugged with cotton wool, sterilized, 

 and allowed to solidify. A fine platinum wire, heated in a flame 

 and allowed to cool, is dipped into the material containing the 

 bacterium in question, and then, after the removal of the cotton- 

 wool plug, is rapidly plunged down through the gelatin to the 

 bottom of the tube and then withdrawn. The plug is reinserted 

 and the tube kept at a temperature suitable for the development of 

 most forms of bacteria, but not high enough to melt the gelatin. 

 If growth takes place at this temperature it occurs either on the 

 surface around the point of entrance of the needle or along the 

 needle track, or in both places, and the appearance of the growth 

 varies remarkably, according to the different species of micro- 

 organisms studied. The second way is to liquefy and pour out a 

 little of the gelatinized material on microscopic slides or on larger 

 plates of glass which have been sterilized by heat. These plates 

 are placed in glass vessels containing moist blotting-paper to 

 prevent drying of the gelatin and to protect them from the dust. 

 After the gelatin has solidified the purified platinum needle charged 

 with the bacteria is drawn rapidly over the surface of the gelatin. 

 Bacteria are sown along the track, grow there, and the whole can 

 be placed under a Microscope and the characteristics of the growth 

 studied with a low power. In the third mode a tube of the gelatin 

 mixture is inoculated with a very minute quantity of the bacteria. 

 The tube is then placed in water at the body temperature to melt 

 the gelatin. When the material has melted it is thoroughly 

 shaken up to difiuse the bacteria through it, and while still liquid 

 is poured out on sterihzed glass plates kept in a moist chamber, as 

 in the former case. Solidification very soon occurs, and the bacteria 

 being caught at various parts of the gelatin grow there in the form 

 of groups or colonies, which can be observed under a low power of 



