MEMOIKS UF THE NATIONAL ACAUEJIY OE SCIENCES. 479 



different organisms, but the result was negative. On couuting, tlie colonies on each side of 

 the plate they were approximately the same. 



Gaslight. — Plates were exposed in the light afforded by an ordinary gas burner and in a dark 

 room, one-half of each plate shaded as before. Ou counting the colonies on i)Iates ilhiniinated two 

 and one half hours, the nanil)er on the protected side was slightly larger than ou the illnminated 

 side. Many other i)lates were made uith varying results, and, finally, some plates exposed for 

 sixteen hours were counted and gave about the same number of colonies on each side. 



Electric li(jht.^-V\at(i!^ were exposed to an incandescent light for varying periods, the longest 

 illumination being for four and a half hours. The colonies numbered approximately the same on 

 botli halves of the plate. 



Comparatii'c absence of lij/kt. — Plates of agar-agar were inoculated, placed in a closet for some 

 hours, and then incubated. The number of colonies did not vary in any noticeable degree from 

 those on the control plate. 



A series of exj)eriments were made with tubes of bouillon inoculated with the different 

 organisms and then inclosed in larger tubes containing tluids of different colors — red, orange, 

 yellow, and blue. These tubes were exposed to sunlight and control tubes were also exposed, one 

 being- placed in water and the other in a tube of water covered closely with black paper. The 

 materials used for making the colored solutions were corallin, chromate and bichromate of 

 potassium, and methylene blue. The solutions were Altered and then i)laced in the sun several 

 days to see if they remained clear. 



The tubes were made in sets of seven for each organism — one for each color — and two control 

 tubes. They each contained 15 cc. of bouillon, were plugged with cotton through which pii)ettes 

 passed, and the whole top was covered with cotton fastened around the tube with a wire. 



From the first set of tubes plates were made each day, but the constant opening allowed 

 contamination, so that a- new set was begun. The number of colonies increased from an average 

 of twenty at the first counting to about one hundred at the end of seven days. Three sets of 

 tubes of bouillon cultures — seven for each organism — were iusolated as before, beginning on 

 December 26. Each tube had received from a dropping apparatus one drop of a bouillon culture 

 of the organism used, had been incubated twenty four hours, and then jilaced in the glass tube 

 containing the colored solution and exposed to the sun. A control plate was made immediately 

 after inoculation and counted ou the following day. After insolation for seven days three sets of 

 plates were made, each set containing five plates, or one for each color used. These plates were 

 incubated for twenty-four hours and then counted — the average of four counts being made. All 

 the plates showed great increase ifi the number of colonies. After eighteen days' insolation new 

 plates were made and counted and still showed an increase in the number of colonies. After 

 fifty-eight days of insolation there was a great decrease in the number of colonies and in many 

 of the tubes molds had appeared which had probably grown through the cotton plugs. Rubber 

 caps were not used for the reason that such a limitation of the amount of oxygen in the tube 

 would not maintain the conditions desired. 



A second and third set of tubes treated in the same manner and capped closely with paper 

 remained undisturbed for a month and then showed contaminations as in the set first noted. 



The chart gives the duration of insolation, the dates upon which plates were made, the colors 

 used for the protecting tluids, and the number of (-olonies found as an average of four counts made 

 in different portions of the plate. 



The results of this study upon the effect of light ou the bacillus tiiphi abdominulis, the bacillus 

 coli communis, and the staplnilococcus pi/oe/enes aureus, in which several hundred plates have been 

 insolated and a considerable number exposed to diffuse light, gaslight, and electric light, are 

 summed up in the following statements: 



Insolation for even the short period of fifteen minutes destroys to some extent all three of the 

 organisms named above. Three to six hours' insolation kills nearly every colony on the plate. 

 The bacillus coli communis is more easily destroyed by insolation than the bacillus typhi abdomtnalis. 

 On jilates covered with black paper, producing complete shadow during the insolation, no destruc- 

 tive effect IS produced, as shown by the number of colonies being the same as on the control, 

 plate. The heat absorbed by the black paper does not destroy the organisms. 



