8 LanksHEAR, Quantitative Absorption Spectra. 



pure absolute alcohol will be found to show signs of 

 absorption at X = 3,000 through a length of 100 mm., and 

 even after careful fractionation over a few drops of 

 sulphuric acid, followed by similar treatment with lime, 

 absorption will show up between A = 2,800 and X = 2,000. 

 Gelatine absorbs ultraviolet light of short wave length, 

 and, since in the ordinary plate the silver bromide grains 

 are embedded in gelatine, they do not receive all the light 

 of short wave length which falls on the plate. The 

 sensitiveness to waves from X = 2,500— 2,000 varies very 

 much, but in all ordinary plates the effect becomes strong 

 as the wave length X = 2,000 is approached. Hence, for 

 measurements in this region, Schumann plates should be 

 used, in which variety a minimum of gelatine is used. 

 Unfortunately, these are not on the market and must be 

 prepared when required. 



Changes in constitution on dilution. This difficulty is 

 not very common, but it manifests itself occasionally in a 

 sudden break in the curve at the point where the solution 

 has been diluted. For an example, see the camphor 

 quinone a — hydrazone and a — semicarbazone. (/. C. S., 

 99, p. 1786.) 



Reading- the Plate. It is usual in reading off the 

 point where a band begins to regard the last line visible 

 as the edge of the band. This works well enough with 

 portions of curves near the visible, but the greater part of 

 the ultraviolet spectrum is diffuse and certain strong lines 

 persist long after weaker ones have disappeared. This 

 trouble can be got over to a certain extent by using 

 another source of light, but it will always remain a cause 

 of uncertainty and additional labour. 



Quantitative vietJwds. An interesting quantitative 

 method has recently been developed by M. Henri {Phys. 

 Zeit., 191 3, pp. 513-516), which will be briefly described 



