GENERAL LABORATORY RULES 89 



this to set, and repeat until the cement becomes level with top 

 of test-tube. When set, a few drops of black lacquer are put on, 

 and a circular cover-glass of about the same diameter as the 

 mouth of tube is placed so as completely to seal it. 



(/;) The following method is useful for preparing 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 

 capsule 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, 

 an<l left in position until the gelatin has solidified. The super- 

 fluous gelatin is now removed, and the glasses sealed first with 

 the orange shellac cement, then with black lacquer. It is now 

 lii i i -lied off by using a circular mask of suitable size. 



The various kinds of solid media used in the cultivation of 

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

 nvated 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, 



