1905.] on the Development of Spectr o- Chemistry . 183 



both reached the same result, viz., that the true measure of refractivity 

 is furnished by the expression 



\n''+ 2) d ' 



Experimental tests showed that this theoretical expression Avas in 

 fact, for all bodies, practically unaffected not only by temperature and 

 pressure, but also by the state of aggregation. 



Chemical tests confirmed the utility of the new optical standard, 

 since the operation of all the laws before mentioned was observed to 

 be even more exact when the new constant was applied. 



§ 23. Moreover, the expression for refraction proved valuable in 

 another respect. It was found to be very suitable for measuring the 

 dispersive power of bodies. 



If n^ and n^ denote the refractive indices for the limits of the 

 visible spectrum, i.e. for violet and for red light, the difference of the 

 refractivities for these end-rays of the spectram : 



(nlj-1^ _ n,^ - 1 \ P 



is the measure of the power of different bodies to disperse light — to 

 broaden out the spectrum. This ratio proved to be constant as 

 regards temperature, pressure, and state of aggregation. 



Gladstone had already observed that dispersion, like refraction, 

 was connected with the chemical nature of bodies. Quantitative re- 

 lations were, however, only obtained when a constant for refractivity 

 had been found. And then from the molecular dispersions of com- 

 p(Uinds the atomic dispersions of their elements were deduced. 



We cannot enter here into the relations which were thus shown to 

 exist between the chemical composition of substances and their power 

 to disperse light. "We need only remark that the case as a whole is 

 analogous to that of refraction. Dispersion is, however, a still more 

 sensitive and more constitutional property, and therefore in many 

 cases it is specially adapted as an aid to research on chemical 

 stnicture. 



YIII. 



§ 24. It only remains to add a few remarks on the applications of 

 spectro-chemistry in science and in practical life. 



I have already shown the principles on which spectro-chemical 

 methods of examination in general can be applied to the solution of 

 scientific problems, to the discovery • of the chemical stnicture of 

 single substances or whole classes of bodies. 



Now there are a large number of substances, some of them arti- 

 ficially built up by synthesis out of their elements, some of them 

 occurring in the vegetable and animal kingdoms, or even in inorganic 

 nature, the structure of which is of remarkable delicacy and insta- 



