ON ORGANIC COMPOUNDS 95 



the organisms record their own destruction, which has again been in- 

 dependently rediscovered by two other sets of investigators at inter- 

 vals of several years. The method consisted in throwing a spectrum, 

 obtained by directing sunlight, or the light of electric arcs in air, 

 through a quartz spectroscope, upon an agar-agar plate sown over 

 with the organism. A number of slots were cut out on the covering 

 lid of a shallow plate like a Petri dish, and some of these were covered 

 with quartz and others with thin glass strips. The spectra were 

 directed through these on to the agar-agar culture. The remainder 

 of the surface was protected by an opaque cover of tinfoil. After 

 exposure for twelve hours the plate was incubated for four days, 

 and then the results were photographed. When the glass was 

 interposed the only area of destruction was that of the blue and 

 violet, but when the quartz only intervened between source of light 

 and organisms the destruction passed far on into the ultra-violet 

 region. A quite similar method was used by Barnard and Morgan, 1 

 who found the lethal action in the ultra-violet so intense that the 

 bright spectral lines in the ultra-violet were mapped out as clearly 

 almost as on a photographic plate. These authors also determined 

 the wave-lengths of the lethal zone of the spectrum, and found its 

 limits to lie between 3287 and 22 65 A. Quite recently the work 

 has been independently repeated with similar results by Browning 

 and Russ. 2 



Some of the authors quoted, especially Marshall Ward, consider 

 the nature of the chemical reaction involved. Marshall Ward 

 drew attention to the oxidising action of blue light upon oils, and 

 considered it probable that the effect might be due to such an oxida- 

 tion of the fat reserve and not a direct action on cell protoplasm. 



It is highly interesting that the chemical reactions upon sub- 

 stances of biochemical origin, here described, are also produced 

 by the same short wave-lengths as those which occasion death of 

 organisms, as is shown by the enormous decrease in activity when . 

 the light is screened by passing through glass or mica. 



Now the substances present in the bodies of the organisms are 

 of those organic types which yield formaldehyde, as shown earlier, 

 when exposed to the action of light vibrations of the shorter wave- 

 lengths. It is well known that formaldehyde in high dilution is 



1 Roy. Soc. Proc., vol. Ixxii., p. 126 (1903); Brit. Med. Journ., November 

 14, 1903. 



2 Roy. Soc. Proc., B, vol. xc., p. 33 (1917), 



