20 GENEKAL MORPHOLOGY AND BIOLOGY 



a certain limit, it by no means follows that death takes place 

 outside such limits. Organisms can resist cooling below their 

 minimum or heating beyond their maximum without being 

 killed. Their vital activity is merely paralysed. Especially is 

 this true of the effect of cold on bacteria. The results of 

 different observers vary; but if we take as an example the 

 cholera vibrio, Koch found that while the minimum temperature 

 of growth was 16° G, a culture might be cooled to -32° .C. 

 without being killed. With regard to the upper limit, few 

 ordinary organisms in a spore-free condition will survive a 

 temperature of 57° O, if long enough applied. Many organisms 

 lose some of their properties when grown at unnatural tempera- 

 tures. Thus many pathogenic organisms lose their virulence if 

 grown above their optimum temperature, and some chromogenic 

 forms, most of which prefer rather low temperatures, lose their 

 capacity of producing pigment, e.g., spirillum rubrum. 



Effect of Light. — Of recent years much attention has been 

 paid to this factor in the life of bacteria. Direct sunlight is 

 found to have a very inimical effect. It has been found that an 

 exposure of dry anthrax spores for one and a half hours to sun- 

 light kills them. When they are moist, a much longer exposure 

 is necessary. Typhoid bacilli are killed in about one and a half 

 hours, and similar results have been obtained with many other 

 organisms. In such experiments the thickness of the medium 

 surrounding the growth is an important point. Death takes 

 place more readily if the medium is scanty or if the organisms 

 are suspended in water. Any fallacy which might arise from 

 the effect of the heat rays of the sun has been excluded, though 

 light plus heat is more fatal than light alone. In direct sunlight 

 it is chiefly the green, violet, and the ultra-violet rays which are 

 fatal. The last-mentioned rays, however produced, have a 

 powerful bactericidal action. By using a quartz spectrometer 

 with a tungsten arc, Browning and Russ have recently shown 

 that the ultra-violet rays with bactericidal action occupy a 

 position in the spectrum at some distance from the visible 

 rays— from 2960 to nearly 2100 Angstrom units. The exact 

 extent varies somewhat in the case of different organisms, but 

 the area of rays in the spectrum effective against any one 

 organism is comparatively sharply marked off. The bactericidal 

 rays have little penetrating power, being completely absorbed 

 by human skin in a thickness of 10 mm. These observers hare 

 also found that those, and only those, rays which are bactericidal 

 to the staphylococcus aureus are absorbed by an emulsion of 

 that organism. Diffuse daylight has also a bad effect upon 



