EXPERIMENTS. 623 



1 : 500 solution of corrosive sublimate, and allow it to 

 remain in contact with the organisms for only one-half 

 the time necessary to destroy them (use an organism 

 for which this has been determined). Then transfer a 

 drop of the mixture to each of three liquefied agar-agar 

 tubes and pour them into Petri dishes. Place them in 

 the incubator and observe them for twenty-four, forty- 

 eight, and seventy-two hours. No growth occurs. How 

 is this to be accounted for ? 



At the end of seventy-two hours inoculate all of these 

 plates with a culture of the same organism which has 

 not been exposed to sublimate, by taking up bits of cult- 

 ure on a needle and drawing it across the jjlates. A 

 growth now results. We have here an experiment in 

 which organisms which have been exposed to sublimate 

 for a much shorter time than necessary to destroy them, 

 when transferred directly to a favorable cultui-e-medium 

 do not grow, and yet, when the same organism which 

 has not been exposed to sublimate at all is planted upon 

 the same medium it does grow. How is this to be ac- 

 counted for? 



Skin-disinfection. — With a sterilized knife scrape 

 from the skin of the hands, at the root of the nails, and 

 under the nails, small particles of epidermis. Prepare 

 plates from them. Note the results. 



Wash the hands carefully for ten minutes in hot 

 water and scrub them during this time with soap and 

 a sterilized brush. Einse them in hot water. Again 

 prepare plates from scrapings of the skin on the fingers, 

 at the root of the nails, and under the nails. Note the 

 results. 



Again wash as before in hot water with soap and 



