GENERAL LABORATORY RULES. 85 



or bacteria through the cotton and the escape of water by evap- 

 oration. " Cool " incubators are often used for incubating gelatin 

 at 21 to 22 C. An incubator of this kind, fitted with a low- 

 temperature Hearson's regulator, is very satisfactory. 



Permanent Preservation of Cultures. This may be conven- 

 iently effected by means of formalin vapour. The cotton wool 

 of the tube containing the culture to be preserved is removed 

 and soaked in formalin (40 per cent formic aldehyde). It is 

 then replaced and covered with an india-rubber cap. After 

 exposure to the vapour in this way for two or three days, the 

 culture will be found to be dead. The final stage in the process 

 is to close the open end of the tube so as to prevent evaporation. 

 Melted sealing-wax or other substance may be poured over the 

 cotton wool, which is first burned off down to the tube, the whole 

 being then covered by an india-rubber cap, or the upper end of 

 the tube may be melted in a Bunsen flame, and thus sealed. In 

 the latter case, tubes longer than those generally employed 

 should be used, so as to leave a longer portion at the top beyond 

 the medium, otherwise in sloped tubes part of the medium is apt 

 to be melted. Cultures preserved in this way maintain their 

 form practically unchanged for several years, though many col- 

 oured growths are apt to lose the colour. Liquefied gelatin 

 usually becomes solidified by the action of formalin vapour, so 

 that the tubes can be freely handled. In the case of agar tubes, 

 any water of condensation should first of all be carefully poured 

 off. 



General Laboratory Rules. On the working bench of every 

 bacteriologist there should be a large dish of i-iooo 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 steriliser for two or three hours, or in the 



