BY R. GREIG-SMITH. 



729 



The method of extracting the toxins consists in taking a quantity 

 of the soil, and shaking it with water, 50 times every five minutes 

 for an hour. With a very toxic soil, this is unnecessary, but with 

 soils in which the toxicity is masked by the nutrients, the full 

 shaking is required. 



Experiment i. 



The extract is then filtered through paper on the filter-pump, the 

 first turbid runnings being returned, and the clear filtrate is fil- 

 tered through a Pasteur-Chamberland F. candle. The first 20-30 

 c.c, are thrown away. Ten c.c. of the filtrate are pipetted into a 

 Freudenreich flask, and seeded with 1 c.c. of a suspension of Bac. 

 prodigiosus, containing a suitable number of cells.* The Freuden- 

 reich flasks are incubated overnight, and counts made by the plate- 

 method; one-fortieth c.c. of several dilutions are smeared on set 

 agar-plates, dried at 37°, and incubated at 28°. 



In the following experiment, an air-dried garden-soil, which had 

 been stored in the laboratory for three weeks, was extracted with 

 distilled water. 



Experiment ii. 



100 grams of dry garden-soil shaken 

 with distilled water. 



75 c.c. 

 100 c.c. 

 150 c.c. 

 200 c.c. 

 250 c.c. 

 Water-control 



1,000 bacteria after 20 hours 

 at 28° became 



420 







123 



202 



5,580 



14 



* A heaped 2 mm. loop of a 20 hours' agar-culture distributed in 10 c.c. 

 of water by blowing, is centrifugalised until the clumps are sedimented ; 

 one c.c. of the supernatant suspension is shaken with 100 c.c. of water, and 

 one to two c.c. of this are shaken with 100 c.c. One c.c. of this last 

 dilution, when added to 10 c.c, and one-fortieth c.c. taken, gives a count 

 of about 200 cells. 



