TRANSACTIONS OF SECTION B, 583 



Section B.— CHEMISTRY. 



President of the Section — Professor Walter Noel Hartley, D.Sc. 



F.R.S., F.R.S.E. 



THURSDAY, SEPTEMBER 10. 

 The^President delivered the following Address : — • 



The ofttimes laborious method of investigating' the relationship of substances by 

 ascertaining how one form of matter can operate upon another, in other words by- 

 chemical reactions, has of late been supplemented by the examination of their phy- 

 sical properties, and has been extended to compounds, both organic and inorganic. 

 In several directions this has led to results of very uncommon interest. Accordingly 

 I propose to ofler a brief account of twenty-five years' experimental work in that 

 branch of chemical physics which deals with the emission and absorption of rays 

 of measurable wave-length, and to review its present position chiefly in relation 

 to the theory of chemistry, indicating where it may be usefully and profitably 

 extended. 



According to Davy,^ Ritter observed chemical action on moist chloride of 

 silver to be different in different parts of the spectrum, slight in the red, greater 

 towards the violet, and extending into a space beyond the violet where there is no 

 sensible light or heat. WoUaston discovered that chemical action was exerted by 

 refracted rays in a region where they were of a higher refrangibility than any rays 

 that were Tisible. Young showed that the invisible rays are liable to the same 

 affections as visible rays. Hence we have the beginnings of spectrum analysis in 

 its chemical relations to terrestrial matter, in the infra-red, the visible, and the 

 ultra-violet regions. 



Everyone is more or less familiar with the subject of spectrum analysis. This 

 was defined by Tait as an optical method of making a diagnosis of the chemical 

 composition of either (a) a self-luminous body, or (6) an absorbing medium, 

 ■whether self-luminous or not. It has now become necessary to enlarge this defi- 

 nition, and I would suggest that it is the study of the composition and the con- 

 stitution of matter by means of radiant energy, and recording in the order of their 

 refrangibilities the rays emitted and absorbed by matter. By tliis modified state- 

 ment the infra-red or so-called ' invisible heat rays,' the visible or ' colour rays,' and 

 the ultra-violet or ' chemical rays ' are included. 



Spectra are of two kinds, emission and absorption spectra. It will be conve- 

 nient if the latter are considered first. 



Absorption Spectra. 



The Infra-red Region. 



Abney (1880) by the preparation of a particularly sensitive form of collodion 

 emulsion containing silver bromide was successful in obtaining very extraordinary 

 results. Such films as he prepared were so sensitive to invisible radiations of 

 long wave-length as to be capable of forming a representation of even a kettle of 



' Chemical PMloiojihy, vol. i. 1812, p. 211. 



