Ml:. CLIVE COTHi:m:i>"N *N" MAVM COTffi*ftfBOU ON THE 



much wider and this api-eared the more surprising since, in other fields of research, 

 the gaseous state has P n,v,-d peculiarly favourable for the discovery of simple 



relations. 



Accumulation of Data. The investigation, of which the present paper forms part, 



was designed to throw light on the cause of these anomalies. 



The first step was to'enlarge the field of the enquiry by the accumulation of data, 

 and with this object we have, either together or in collaboration with others, deter- 

 mined and redetermined the refraction and dispersion in the gaseous state of fourteen 

 elements and ten compounds within the limits of the visible spectrum, and the 

 refraction of six elements and four compounds for a single wave-length. 



The numU-r is still far too small. Many interesting compounds remain to be 

 invest igated. But, as the present instalment of work has occupied nearly two years 

 and has led to certain definite conclusions, it seems better to publish it rather than to 

 await the addition of more difficult and, perhaps, less instructive examples. 



( '/,,./.-. ,/' ( 'ntiHtundx. In selecting the compounds to be examined we have been 

 guided by the principle that the molecule should consist of as few atoms as possible, 

 and that the refraction and dispersion of each constituent should be measurable in 

 the gaseous state. It is unfortunate that the list of compounds which comply with 

 these requirements is very short. All compounds of carbon are excluded. Of those 

 substances which are dealt with in the present paper, the most important are the 

 halogen acids, which form a regular series of simple diatomic molecules. Steam, S0 2 , 

 and HjS form an interesting cycle, and the two oxides of nitrogen are very instructive 

 since the eonstituents are the same in both cases.* Ozone is remarkable as an 

 example of the effect of polymerization on the refractivity. 



l-'.j- 1 >, .<>/// of Results. The choice of a formula for the expression of results is of 

 fundamental importance. Previous workers on the subject of gaseous refractivities 

 have almost invariably used that of CAUCHY, with two terms or three, according to 

 the degree of accuracy of their figures. But this formula is not based on modern 

 physical theory. Moreover, we have shown in a previous paperf that, when only two 

 constants are used, it is inadequate to express the experimental results, even in the 

 \ isible spectrum ; while if a third constant and term involving I/A 4 is introduced, the 

 shape of the dispersion curve cannot be easily grasped on inspection of the figures. 

 For these reasons we have abandoned this formula, and have used, tentatively, a 

 formula of SKI.LMKYKKS type, l = M -i (approximately) = N 



yl LJl^ ff" 



It is unnecessary to defend the adoption for gaseous indices of this expression, 

 been widely used for solids, and is in general outline established on theory 



The exwnin.tiou of NO, and N A haa been postponed on account of its difficulty, but promises to be 

 1 important owing to the association which characterises it 



Hspersion of Air, Oxygen, Nitrogen, and Hydrogen and their Relations," 

 , Mid M. GtTHBwmtoN, Roy. Soc. Proc.,' A, vol. 83, p. 151, 1909. 



