E. M. Doidge 39 



A series of thinly sown plates were exposed to direct sunlight, any- 

 undue heating being prevented by placing over each a glass basin 

 containing about 2 cm. of a 2 % solution of potash alum. The number 

 of colonies on plates exposed one to four hours was not appreciably less 

 than the number developed in the control plates; there was a slight 

 diminution in numbers in those exposed for five hours. This experiment 

 was conducted in winter during the month of July. 



Four cover glasses on which a film of bacilli had been dried were 

 exposed to bright sunlight for one, two, three and four hours, then 

 dropped into tubes of nutrient broth. Those exposed for one and two 

 hours clouded the broth in 24 hours; those exposed three and four 

 hours in 48 hours. 



Thermal Relations. 



The apparatus used for the determination of the thermal death 

 point and figured below was adapted by Mr Ensor of the Public Works 

 Department from a Hearson vacuum embedding bath intended for 

 working with gas; it answers the purpose admirably, and regulates 

 within one-tenth of a degree. 



I am indebted to Mr Ensor for the following brief description of the 

 apparatus and for the figure explaining the electrical connections : 



Constant temperature water bath. The apparatus consists of the 

 water bath of a Hearson vacuum embedding bath (A, diagrams 1 

 and 2). This is electrically heated by means of two eclipse elements 

 (L, diagram 3) fixed to the bottom of the bath. The elements are 

 connected in series on 250 volts. 



For automatic regulation of temperature a switch (B, diagrams 1, 2 

 and 3) actuated by a capsule (J, diagram 3) is provided, this makes or 

 breaks the heating circuit at the required temperature. A lamp (K, 

 diagram 3) is joined across the switch contacts to prevent sparking. 



There is also a tumbler switch (C, diagrams 1, 2 and 3) by means of 

 which the temperature can be controlled by hand if desired. 



The water is continually stirred by means of a paddle (D, diagrams 

 1, 2 and 3) driven by a small motor E fixed above the bath. 



In order to cut down the speed of the motor to about 60 revolutions 

 per minute a lamp (H, diagram 3) is connected in series. The speed 

 can be varied by placing lamps of different candle power in the lamp 

 holder. 



The motor is started and stopped by the switch F. 



