GENERAL LABORATORY RULES 83 



(b) The following method is useful for preserving plate cultures. 

 Instead of making the cultures in Petri's capsules, use ordinary 

 watch-glasses. The watch-glass is sterilised in a Petri's capsule, 

 and the inoculated medium is poured out into the watch-glass, 

 allowed to solidify in the usual way, and left in the Petri's cap- 

 sule until the colonies of growth have developed. The watch- 

 glass is now removed from capsule and a layer of the preserving 

 gelatin medium (1), to which have been added a few drops of 

 strong formalin, is allowed to spread over the surface of the culture 

 medium. When the layer is solidified the watch-glass is filled up 

 with the same, and a clean square or oblong piece of glass 

 (which of course should be of slightly larger diameter than the 

 watch-glass) is now carefully placed over watch-glass, care being 

 taken that no air-bells are formed. The edge of watch-glass 

 should be closely applied to the glass cover and left in position 

 until the gelatin has solidified. The superfluous gelatin is now 

 removed, and the glasses sealed first with the orange shellac 

 cement, then with black lacquer. It is now finished off by 

 using a circular mask of suitable size. 



The various kinds of solid media used in the cultivation of 

 bacteria, such as blood serum, potato, bread paste, etc., can be 

 treated in the same manner with excellent results. 



General Laboratory Rules. On the working bench of every 

 bacteriologist there should be a large dish of 1-1000 solution of 

 mercuric chloride in water. Into this all tubes, vessels, plates, 

 hanging-drop cultures, etc., which have contained bacteria and 

 with which he has finished, ought to be at once plunged (in the 

 case of tubes the tube and plug should be put in separately). 

 On no account whatever are such infected articles to be left 

 lying about the laboratory. The basin is to be repeatedly 

 cleaned out. All the glass is carefully washed in repeated 

 changes of tap water to remove the last trace of perchloride of 

 mercury, a very minute quantity of which is sufficient to inhibit 

 growth. Old cultures which have been stored for a time and 

 from which fresh sub-cultures have been made ought to be 

 steamed in the Koch's steriliser for two or three hours, or in the 

 autoclave for a shorter period, and the tubes thoroughly washed 

 out. Besides a basin of mercuric chloride solution for infected 

 apparatus, etc., there ought to be a second reserved for the 

 worker's hands in case of any accidental contamination. When, 

 as in public-health work, a large number of tubes are being 

 daily put out of use, they may be placed in an enamelled slop- 

 pail and this when full is placed in the steam steriliser. 



A white glazed tile on which a bell-jar can be set is very 



