CHAPTER VII
ROSCOE AS AN INVESTIGATOR
The character of Roscoe’s scientific work may also be said to have been entirely moulded by his Heidelberg training, and Bunsen’s influence may be traced through it to the last. So completely was this the case that consciously or unconsciously he seemed never to contemplate attacking any problem that would not have appealed to, or have been appreciated by, Bunsen.
His first research was undoubtedly suggested by Bunsen. As already stated, it resulted in the classical investigation on the laws regulating photochemical action. It was already known that a mixture of equal volumes of hydrogen and chlorine on exposure to light lost its characteristic colour, and was converted into hydrochloric acid, readily soluble in water; and Bunsen conceived the idea of making this reaction the basis of a method of measuring the relative amount and activity of those light-vibrations which are mainly concerned in effecting chemical change. As a matter of fact the idea was not new, for, unknown to Bunsen, it had already been adopted by Draper, of New York, who had, as he states in his paper in the Philosophical Magazine for December 1843:
invented an instrument [based upon the same reaction] for measuring the chemical force of the tithonic rays, which are found at a maximum in the indigo space, and which from that point gradually fade away to each end of the spectrum.
It perhaps says little for the assiduity with which young Roscoe read the original chemical literature of his time that he should only have knowledge of Draper’s remarkable papers some thirteen years after they were published in the English journals. But it is only due to him to say the chemical students of University College in those far-off days had fewer opportunities of access to original literature than they now enjoy.
Be this as it may, Roscoe’s discomfiture at being thus anticipated was of no long duration.
Do not (wrote Bunsen) let your discovery of Draper’s work disconcert you.… It appears to me that the value of an investigation is not to be measured by whether something is described in it for the first time, but rather by what means and methods a fact is proved beyond doubt or cavil, and in this respect I think that Draper has left plenty for us to do.
After many fruitless attempts they succeeded in constructing an apparatus in which the defects of Draper’s “tithonometer” were obviated, and by which not only accurate comparative determinations could be made, but which enabled them to reduce the chemical action of light to absolute measure. They showed by means of it that the amount of chemical action produced by light from a constant source varied inversely as the square of the distance. They studied more accurately the phenomena of photochemical induction, discovered by Draper, the causes which determine its occurrence, and the laws which regulate the chemical action of light after the induction is completed. They proved that the absorption of the chemical rays in passing through a medium varies directly as the intensity of the light, and that the amount transmitted varies proportionately with the density of the absorbing medium. It was found that for a given amount of chemical action effected in the mixture of chlorine and hydrogen an equivalent quantity of light is absorbed, and that the coefficients of extinction of pure chlorine for chemical rays from various sources of light are very different. They established a general and absolute standard of comparison for the chemical action of light, and sought to determine the quantitative relations of the chemical action effected by direct and diffused sunlight, and to investigate the laws which regulate the distribution on the earth’s surface of the chemical activity emanating from the sun. They also measured the chemical action of the constituent parts of the solar spectrum. The action on the sensitive gas showed the existence of several maxima of chemical intensity in the spectrum. The greatest action was observed between the lines G in the indigo and H in the violet, whilst another maximum was found to be near the line I in the ultra violet. Towards the least refrangible end of the spectrum the action became imperceptible about the line D in the orange, but at the other end of the spectrum the action was found to extend as far as Stokes’s line U, or to a distance from the line H greater than the total length of the ordinary visible spectrum.