380 



Dr. P. F. Frankland. 



[June 18, 



glass plate, when, after the lapse of a few days, the colonies derived 

 from the individual centres of life can be counted by means of a lens, 

 and from this the number present in a cubic centimetre or any other 

 volume of water can be calculated. The following is a description of 

 the exact mode of procedure adopted : — • 



Determination of the Number of Micro-organisms in Water. 



The nutritive gelatine was prepared thus : — 1 lb. of lean meat is 

 finely minced and then infused with ^ litre of water for 1 — 2 hours, 

 the solid part being then strained off through linen. 100 grams of 

 white French gelatine is allowed to soak in another \ litre of water, 

 and to this the extract of meat obtained above is added. The whole 

 is now boiled for a few minutes to complete the solution ; 10 grams of 

 peptone and 1 gram of common salt are then added and dissolved. 

 The mixture so obtained gives an acid reaction, which is carefully 

 neutralised with carbonate of soda. The liquid is now clarified by 

 beating in the contents of two or three eggs along with the broken 

 •shells, the whole being briskly boiled for a few minutes. The coagu- 

 lated albumen rises to the surface and carries with it the other solid 

 particles in the liquid. On then straining through linen, an almost 

 clear liquid is obtained, which is finally clarified by passing through 

 filter-paper kept hot by means of a water-jacket. On cooling this 

 liquid, it sets to a yellowish-brown transparent jelly. Whilst still 

 liquid it is poured into clean test-tubes, so that each of these contains 

 2 or 3 c.c. The test-tubes are tightly plugged with cotton-wool, and 

 ihen sterilised by steaming them for half an hour, on three consecutive 

 days. Tubes thus prepared were found to keep for an indefinite period 

 •of time. The glass plates destined to receive the film of gelatine were 

 well washed, and then placed in a copper box provided with a tightly 

 fitting lid, the whole being sterilised by heating for at least three 

 hours to 150° C. 



The glass dishes in which the gelatine plates are placed during the 

 development of the organisms were always well washed, and then rinsed 

 with a 2 per cent, solution of mercuric chloride immediately before use. 

 Some of the same solution of mercuric chloride was poured into the 

 bottom dish, so as to preserve the internal atmosphere saturated with 

 moisture, and the two dishes were always placed in a porcelain tray 

 containing a solution of mercuric chloride, so that the interior of the 

 dishes was disconnected from the outside air by means of a mercuric 

 chloride seal, whereby all ingress of organisms from the atmosphere 

 was prevented during incubation. The dishes also contained a small 

 glass tripod, bearing a glass plate, the surface of which was carefully 

 levelled by placing the dishes on a table provided with three screws. 

 When the apparatus is thus prepared, the sterilised plate is rapidly 

 transferred to the levelled plate, the gelatine is carefully melted in 



