THE TROPICAL SUNLIGHT. 11 



are a few by Elster and Geitel 1C giving us an indication of what the fall 

 of voltage would be in northern climates. They found, in Vienna on a 

 foggy day, a voltage of 2.77, in clear weather, 8.58, but on a day when the 

 •sky was half overcast, 13.67. These authors ascribe the phenomenon 

 to radio-activity, but our results in Manila, where radio-active phenomena 

 are not especially prominent, would lead to the conclusion that the air 

 is ionized by sunlight. The presence of this ionization in so great a 

 degree in our atmosphere would indicate a condition of the solar spectrum 

 which might well account for many of the so-called excessive effects 

 which have been observed. 



THE EFFECT OF SUNLIGHT ON MICROORGANISMS. 



A great mass of literature exists on this subject, but the later publica- 

 tions of Hertel 1T have brought the study down to such a clear basis that 

 it seems scarcely necessary to enter into the work of previous authors with 

 any detail. Suffice it to say, it is a fairly well established fact that 

 bacteria and even their spores are destroyed by sufficient exposure to 

 the sunlight. Of course, the time necessary for this effect varies greatly 

 with the latitude and the degree of insolation, and naturally the errors of 

 observation are very large. 



The general method of procedure has been to expose plate cultures or tubes 

 to the direct action of the sunlight during fixed hours, placing controls in the 

 incubator, and covering either portions of the plates or tubes," or their contents 

 where needed, by tin foil. Of course, in the majority of published researches, 

 care was taken to exclude heat action by proper precautions. Different colors 

 were produced by colored plates or solutions, in some instances spectroscopically 

 examined. But few of the authors have used quartz prisms and lenses to give 

 a spectrum. It is well known that glass absorbs the ultra-violet of the spectrum. 



Hertel undertook his experimental work not by employing the entire 

 sunlight, or by blending with colored glasses, but by the spectrum of the 

 sun from the ultra-violet region, using an apparatus the parts of which 

 were constructed of quartz, testing the action of the light within different, 

 but sharply defined regions of determined wave length. The first source 

 of light employed was the bright line from magnesium at A=280 /x/n. 



Bacteria and vibrios (B. coli, B. prodigiosus, the vibrio of cholera, B. typhosus 

 and others) in the first moments of illumination by this portion of the spectrum, 

 increased in motility, that is, gave evidence of stimulation, but after a few 

 seconds there was a retardation and finally complete rest. The bacteria of decay 

 also were markedly affected, with final death. Especial experiments demon- 

 strated that the light had no effect on the culture media, so that the sterilization 

 resulted because of the action of the radiant energy on the organisms alone. 



10 Ann. d. Phys. (IV), 2, 425. The authors used an instrument of identical 

 form with our own. 



17 Ztsohr. f. allg. Physiol. (1904), 4; (1905), 5; (1906), 6; Rev. in Biol. 

 Centralbl. (1907), 27, 510. 



