This article describes work done with the spectrophotometer with the view of clearing up four different points arising out of the earlier articles. At the time it was commenced, the hope of finding a mathematical connection between absorption spectra and molecular structure had been given up, at least so far as aqueous solutions were concerned, and attention was being concentrated on some chemical questions arising out of the work. The spectrophotometer and methods were the same as had been used in Nos. II., III., VI., VII., and VIII. of the series.