64 DUDLEY MEMORIAL VOLUME 



therefore, to, revolve fifty or more cultures simultaneously, but at five differ- 

 ent rates. The apparatus runs continuously, day and night, and requires 

 to be wound only once in thirty hours. 



After experience and reflection, I concluded that the only constant and 

 uniform force at my disposal for driving my clock-work was gravity. This 

 is represented by discs of cast iron and of lead, amounting to a weight of 

 two hundred and fifty (250) pounds approximately 113.4 Kilos which is 

 hung from a pulley running on a wire cable, the end of which is fastened to 

 a beam in the ceiling of the room. The clock is wound by pulling up the 

 weight nearly to the ceiling, the wire winding upon a drum revolved by a 

 crank. 



The material and methods of culture have undergone nearly as many 

 changes since I began as the method of revolving the cultures. With the 

 exception to be noted below, I still use small crystallizing dishes. These are 

 about 7 cm. diameter and 3 cm. depth. I have had small square tiles of 

 porous flower-pot clay especially made. These fit into the bottom of the 

 dishes, and, as their upper surface is smooth, the least possible shadow is cast 

 by one part upon another. These porous tile are first boiled in distilled 

 water, to extract, as completely as may be, the soluble matter which they may 

 contain. To this end I boil the tile in distilled water for hours, using three 

 or four waters for this washing and leaching process. The tiles are then 

 allowed to drain and dry. Meanwhile Knopp's Solution is made as follows : 



SOLUTION A SOLUTION B 



KNOs 2grs Ca(NO3) 2 8 grs 



MgSO4 2 " Aq. dist 3000 cc 



KsHPCU 2 



Aq. dist 1000 cc 



To 1 part of A 3 parts of B are added and the mixture boiled for fifteen 

 minutes in a cotton-plugged flask. The resulting precipitate is either filtered 

 off or allowed to settle in the bottom of the flask. At all events, the clear 

 liquid is poured into the culture dishes to about three-quarters the depth of 

 the porous tiles. The dishes are covered with the lids or bottoms of Petri 

 dishes of suitable size. They are now steam sterilized for an hour or more 

 and are allowed to cool over night in the sterilizer. As it is necessary to 

 prevent light from falling otherwise than from the side upon the plants to be 

 cultivated, the lids of the dishes are given, after cooling, a smooth continuous 

 coat of dull black "Japalac," an inexpensive and quickly drying varnish easily 

 applied. 



It may be wondered why I did not use Plaster of Paris poured into the 

 crystallizing dishes and allowed to set in them as molds. These would have 



