1885.] Studies of Disinfectants by New Methods. 267 



The only special apparatus used requiring description is the " drop- 

 bottle " and the " rings and plates." 



The Drop-bottle. — The figure represents its shape and size, the 

 capacity is about 25 c.c. The stopper is hollow and terminates in 

 a pipette ; it has a pin-hole at a, which can be closed by the 

 finger. 



The Glass Plates and Rings. — The glass plates were 4 by 2 inches 

 square, the rings 4 inches in diameter, ^ inch thick, and \ inch high. 

 The plates had a ground surface the size of the ring thickness ; the 

 rings were cemented to the plates in the following manner. After 

 heating the rings and plates in a hot air oven for many hours a little 

 peptone gelatin was run on to the ribbon of ground surface, the ring 

 adjusted, and the whole allowed to cool in a glass chamber formed by 

 a small dish covered by a slightly larger one ; at the bottom of the 

 dish was some filter-paper moistened with a solution of mercuric 

 chloride. The plates were not used until the gelatin cement had 

 perfectly set. I should also add that the plates were ruled by means 

 of a diamond into squares for the purpose of easy enumeration. 



Solid substances, such as ferrous sulphate, were weighed and dis- 

 solved in definite quantities of the sewage ; in other cases solutions 

 of known strength were mixed with the sewage. The time during 

 which the disinfectant acted was, as a rule, twenty-four hours. 



The method of cultivation was as follows : — A small quantity, 

 whether of diluted or disinfected sewage, was transferred to the 

 previously cleansed and sterilised drop-bottle, the bottle and its 

 contents carefully weighed, then by means of the pipette stopper 

 one or two drops spotted on to the surface of the glass cell formed 

 by the plate and ring already described; the weight of the drops was 

 ascertained by reweighing the drop-bottle. 



Ordinary nutrient gelatin liquefied at a gentle heat was run from 

 a Lister flask into the glass cell, and mixed equally with the drops by 

 inclining the plate in different directions. During these several 

 operations dust was excluded as far as possible by covering the glass 

 cell by a second glass plate, merely shifting the plate sufficiently on 

 one side to allow the insertion of the nozzle of the Lister flask or the 

 point of the pipette. The cells thus charged were placed in the 

 moist chamber ; the gelatin rapidly set, and at the end of from three 

 to five days the colonies of growth were counted in the usual way 

 and their general nature determined. 



The weight of the drop or drops taken varied from 20 to 100 mgrs., 

 the gelatin in which the drop was cultivated from 15 to 20 grams, 

 so that the minute quantity of disinfectant contained in the drop 

 itself was diluted from 200 to 1000 times. This amount of dilution 

 with the comparatively weak percentages of disinfectants used would 

 reduce the action of the disinfectant on the gelatin, the cultivating 



