GENERAL LAB OR A TOR Y R ULES. 9 1 



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 em- 

 ployed 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 coloured growths are apt to lose 

 the colour. Liquefied gelatine 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 con- 

 tained 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. In making examinations 



