SPECTROPHOTOMETRY AND MEDICINAL LIQUID PARAFFIN 9 The largest increase in absorption intensity was always found at 2490 A, even in the case of extremely pure liquid paraffins whose spectra, showed no band structure (Fig. 5). A similar type of increase of absorption intensity, though at a slower rate, was found in the neighbourhood of 2710 A (Fig. 6). / X"•-• 5 a[ler 575 hrs. in 0.09[ X ', 0,04 ••• •'x• 2,400 2 5• 256• 2 7• 2,8 Change in ultra-violet absorption on irradiation by mercury vapour lamp. Fig. 5. The largest increase in absorption intensity as the result of ultra- violet irradiation is always around 2490/• irrespective of the initial quality of the liquid paraffin.

l0 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.05 DISTANCE FROM LIGHT SOURCE 45 cm. . X. 90 •m. © (• 135 c• [] [] 500 1,000 hrs. 1,500 2,0•00 2,500 Fig. 6. Increase in absorption intensity, AE, with the time of ultra-violet exposure of three samples of material No. 8 at distances of 45 cm., 90 cm., and 135 cm. from the light source. The crosses indicate in each case the Alg values at 2490 ]iU., and the dots show the corresponding Alg values at 2710 Quantitatively, the rate of increase of the absorption intensity as the result of ultra-violet irradiation is governed by the value of the absorption intensity at 2710 A of the unexposed material. For example, for liquid paraffins with E(1%, 1 cm.)-- values of 0'0058, 0-0268, and 0.049 at 2710 A, the initial rates of increase of E(1%, 1 cm.) at 2490 A owing to ultra-violet irradiation were 0'00015, 0.00043, and 0.00062, respectively. This suggests that the "Group A" compounds are the primary absorbers. It is of interest that there is no indefinite increase in absorption intensity either at 2490 A or at 2710 A with ultra-violet irradiation. After about 200 hours' exposure the rate of increase becomes noticeably slower. Saturation is almost reached after 800 hours, and when the exposure is continued for about 2,500 hours the increase in absorption intensity at 2710 • catches up with that at 2490 A.