SPECTROPHOTOMETRY AND MEDICINAL LIQUID PARAFFIN l l MECHANISM OF THE AGEING REACTION The "Group A" compounds are the primary absorbers of radiation. They transfer the thus acquired energy apparently by a collision process to the "Group B" compounds, which then undergo a chemical reaction. The appearance of absorption bands at 2.85/• and 5.85/• (which are charac- teristic for OH-- and C: O-- stretching vibrations) in ultra-violet irradi- ated liquid paraffin suggests an oxidation reaction, so that the availability of oxygen during the irradiation must be of influence on the change of the ultra-violet spectrum of exposed liquid paraffin. Indeed, experiments on three portions of the same liquid paraffin, one of them in air, the other in an atmosphere of nitrogen, and the third in an evacuated quartz vessel, proved that the increase in ultra-violet absorption intensity with exposure time was similar in the three cases, but that the rate of increase was largest in the presence of air and smallest when the vessel was evacuated (Fig. 7). olo oo A[ x x x/• ••• o o r4 •) o o o o• Quartz vessel evacuated I & II I 500 !,000 !,500 2,000 2,500 t•rs Fiõ. 7. Increase in absorption intensity, AE, as function o• the ti•e of ultra-violet irradiation o• three samples o[ material •o. 9, x denotes the AE values at 2490 J•O. ß denotes the AE values at 2710 J•U.

12 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The absorption band at 5.85/• appeared first in the sample which was in contact with air, then in the sample which was kept under reduced air pressure ( 1 mm. Hg), and lastly in the sample which was blanketed with nitrogen (Fig. 8). 02• o1( AD Fig. 8. Increase in the optical density at 5.85 /• as the result of ultra-violet irradiation of medicinal liquid paraffin: ß in air, X in an evacuated quartz vessel, (•) under a blanket of nitrogen. "Stable" liquid paraffins can be exposed in glass vessels to a mercury. lamp for several hundred hours without suffering any appreciable change of their ultra-violet absorption spectra. There is no reason to believe that the mechanism of ageing of liquid paraffin in glass bottles which are displayed in shop windows is essentially different from the above described mechanism. Obviously the transmission of radiation in the region of 2710 • through glass is very low, and indeed the initial rate of increase of absorption intensity was found to be smaller by a factor of about 1000 when liquid paraffins of varying degrees of stability were exposed in glass bottles in diffuse daylight.