92 



REPORTS ON THE STATE OF SCIENCE. 



in the direction of the visible region ; on plotting the thickness against 

 the limit of transmission ('/^) a curve D C E of general absorption is 

 obtained in shape resembling roughly a portion of a parabola. In 

 addition to this general absorptive power, many liquids have the 

 property of absorbing specifically light of a particular wave-length; but 

 this absorption (unlike the line-spectra of vapours) covers a consider- 

 able range of wave-lengths on either side of the maximum absorption ; 

 this range increases as the thickness of the liquid is increased, so that 

 the curve A B F of ' specific ' absorption is somewhat of the shape of 

 a narrow parabola. As the curve of general absorption intersects the 



A,E 



FREQUENCY /a. 



curve of specific absorption at different distances on the axis of ' thick- 

 ness,' the arms of the parabola are unequal in length, the complete 

 curve being generally of the form A B D. The point B is the 

 ' head ' of the band ; the thickness at which it occurs under given 

 experimental conditions is a measure of the ' penetration ' of the band, 

 whilst the range of thickness from B to is a measure of its ' per- 

 sistence. ' Both quantities are conveniently measured in terms of ' log. 

 thickness in millimetres of millinormal solution,' e.g., if B = 10 mm. 

 and C = 100 mm. then 



log penetration = 1*0 



log persistence = 2 -0 — 1 -0 = 1 -0. 



It will be noticed that the depth of penetration of the band is the best 

 measure of the intensity of the local absorption, whilst the persistence, 

 which depends on the relationship of the local to the general absorption, 

 is likely to be greater when the head of the band occurs at a low than 

 at a high frequency. 



In classifying absorption-spectra it has been customary to draw 



