632 VERHOEFF AND BELL. 



and produce heat, certain particular wave frequencies fall into step, 

 as it were, with the oscillation periods which depend on the mo- 

 lecular structure, and so break up the molecules when the energy 

 absorbed is sufficient. The particular kind of radiation which pro- 

 duces this direct action depends on the character of the molecules. 

 Thus, for instance, the green modification of silver bromide is readily 

 broken up by radiation of wave length as great as 1 /jl, while it requires 

 radiation of double this frequency to affect ordinary silver bromide, 

 and the molecules of living protoplasm begin to break up only when 

 the wave length is down to about 300 yu/x as we shall show. But most 

 chemical compounds are unaffected by any practical amount of 

 radiation which may fall upon them except as they may be heated to 

 the point of decomposition. Any effect which is due to radiation 

 is in the last analysis dependent on the absorption of that radiation, 

 in that there is involved a transfer of energy to the molecules or their 

 parts in order that they may be heated or shaken apart. Every sub- 

 stance absorbs radiant energy in greater or less degree, and the amount 

 of absorption bears a definite relation to the thickness of the body as 

 well as to the particular wave length of the incident energy. Certain 

 substances, like fluorite and to a less degree quartz, let pass with very 

 little obstruction radiation from far into the infra red to wave length 

 200 iJLiJL. Water absorbs the longer wave lengths of the infra red up 

 to about 1.2 /i powerfully, and transmits nearly everything else up to 

 the extreme ultra-violet, while pure air, generally speaking extremely 

 transparent, produces some small but sharp absorption in the visible 

 spectrum and completel}^ wipes out the extreme ultra violet. But 

 whatever the wave length, the law connecting the absorption of energy 

 with thickness of the medium is extremely definite. If a layer of unit 

 thickness transmits a certain fraction T then any other thickness x will 

 transmit a fraction T'^ of the incident energy. Thus, if a substance 

 transmits badly, leading to a low value of T, very little energy gets 

 through the outermost layers, while if it be fairly transparent a con- 

 siderable amount of energy penetrates deeply. For example, a cer- 

 tain Jena glass transmits violet light through 1 mm. of thickness 

 with only a small fractional per cent of loss. It transmits the same 

 light througli a cm. of thickness with the loss of only 2%, while 

 near the extreme ultra violet of the solar spectrum it still transmits 

 a little over 90% for a mm. of thickness, but barely 3S% through a cm. 

 Where, therefore, radiant energy falls on a solid upon which through 

 absorption it produces powerful chemical action, the immediate effect 

 will be almost wholly superficial, and only by prolonged and intense 



