This study originated from the investigation on cytochrome, which is a widely distributed, intracellular, respiratory pigment. It was found in the course of this investigation that many problems concerning this pigment would be more easily approached, or even solved, when we have succeeded in elucidating certain obscure points concerning some of the better-known and simpler compounds such as turacin and hæmatin. It was found, for instance, of paramount importance to obtain a better understanding of the relationship between the physico-chemical state of the pigment and its absorption spectrum. It will be shown here that the definite change in the absorption spectrum of a substance, which passes from a state of a solution into that of a colloidal suspension, explains a number of phenomena which have been previously observed but not understood. The combination between the reduced or oxidised hæmatin and various nitrogen compounds, in other words the relationship between such compounds as hæmatin, hæm, hæmochromogen and kathæmoglobin will also be considered. This will be shown to have an important bearing on the study of cytochrome. The methods used in this investigation do not differ much from those already described in my study on cytochrome (1925, pp. 313-314). Two instruments have been used for this study: the microspectroscopic ocular of Zeiss and the Hartridge reversion spectroscope, the latter having been used both in the ordinary way and as a microspectroscope. The sources of light have varied according to requirement, and were either ordinary filament electric bulbs (50 candle-power), pointolite lamp, Nernst lamp or a small arc. The latter was very useful in the spectroscopic examinations of turbid fluids and precipitates. We can hardly overestimate the importance of the last-named method, for a number of essential spectroscopic properties of the pigments can be revealed only by the study of their solutions during the process of precipitation.
Comparative study of spectroscopic properties of some indium, tin and antimony compounds