386 CHAMBERLAIN AND VEDDER. 



bouillon mixed with double the quantity of distilled water. During this period 

 the flask was handled with care so that the fluid was never shaken, and the amoebae 

 that developed were found in countless numbers lying in the surface film of the 

 bouillon. This flask was then used as a stock culture from which all the inocula- 

 tions of an experiment were made, and a single large loopful taken from this 

 surface film was used for each inoculation. 



B. Plates. — The lamp already referred to is in the form of a small arc and 

 the rays, both visible and invisible, radiate from this arc in every direction. The 

 radiations diminish in intensity as the square of the distance from the source, 

 and therefore it was desirable to make the exposures quite close to the lamp, 

 approximately 10 centimeters from it. B\it it seemed probable, if ordinary-sized 

 Petri dishes were used at this distance, that the outer margins of the plate 

 might be beyond the sphere of effective intensity of the rays, and that for this 

 reason some of the amoebae might not be killed. In order to avoid this difficulty 

 we used special plates consisting of aluminum ointment boxes 2.5 centimeters 

 in diameter and with tightly fitting covers. These boxes were sterilized in the 

 autoclave, filled with plain sterile agar and covered with their lids. Tliis kind 

 of plate avoided the difficulty just mentioned, and was very convenient to handle, 

 took up less space in the incubator, and required much less agar, points by no 

 means beneath consideration when it is a question of making several hundred 

 exposures. 



C Method of making exposures. — ^The inoculated plate was uncovered and placed 

 directly under the center of the lamp. The cover was turned upside down and 

 placed beside the plate in order that it should also be exposed to the rays 

 with a view to killing any amoebae that might have contaminated it accidentally. 

 The switch controlling the current operating the lamp was then closed and 

 the appearance of the light timed with a watch. After the desired number of 

 seconds of exposure, the switch was opened. We were thus enabled to time our 

 exposures accurately. As soon the exposure was completed the cover was replaced 

 on the plate. 



This method was quite satisfactory except for one particular. We know 

 that it is necessary for the lamp to become warmed up before it is capable of 

 producing its maximum output of ultra-violet rays. This occurs a few seconds 

 after the current is turned on, in which time the lamp becomes quite hot. 

 It is probable that this temporary lack of efficiency in the apparatus when 

 the current is first turned on explains why some of our cultures were not 

 killed by exposure for a period of five seconds. This will be referred to again 

 later. 



In order to prove that the amoebae and bacteria were killed by the ultra-violet 

 rays, and not by heat generated by the lamp, we exposed a thermometer 10 

 centimeters from the lamp, that being the nearest point at which the cultures 

 were ever placed. After a forty-second exposure the thermometer had risen 

 to 50° centigrade. Thus it is shown that although the lamp itself may become 

 very hot, sufficient heat to injure bacteria or amoebae is not radiated to a distance 

 of 10 centimeters in a period of forty seconds. Only a slight sensation of 

 warmth is felt if the hand is exposed close to the lamp, although a severe burn 

 might be caused if the lamp itself were actually touched. 



Experiment I. — In this experiment plates made as described above 

 were exposed to the ultra-violet rays immediately after they were ino- 



