Report on the Bacteriology of Water. 



305 



black paper, so that, on exposure to the sun, only the cut-out figure 

 or letter allowed the solar rays to reach the agar. 



Such plates were then exposed to the direct rays of the October 

 sun for from two to six hours; or they were placed on the ring of a 

 retort-stand, stencil downwards, and the sun-light reflected upwards 

 from a plane mirror below. 



After the insolation, these plates were incubated for at least forty- 

 eight hours at 20° C, and on removing the wrappers the colonies of 

 anthrax were found densely covering all parts of the plate except the 

 area — a letter or cross, &c. — exposed to the sun-light. There, how- 

 ever, the spores were killed, and the agar remained perfectly clear, 

 showing the form of a sharp transparent letter, cross, &c, in a ground- 

 work rendered cloudy and opaque by the innumerable colonies of 

 anthrax. 



Experiments proved that this was not due to high temperature, for 

 a thermometer with its bulb next the insolated glass rarely rose 

 beyond 14° to 16° C, and never beyond 18° 0., and even if the 

 thermometer did not record the temperature inside the plate, this 

 can scarcely have been much higher. 



As long as this latter point remained uncertain, however, the 



Fig. 1. 



