408 



Dr. J. H. Gladstone. 



[June 16, 



It will be seen by a glance at this table, that the dispersion equiva- 

 lents of the elementary substances are not in proportion to their 

 atomic weights, or, in other words, that they have different specific 

 dispersive energies. Thus the analogous elements, sulphur anil 

 oxygen, are strongly contrasted in this respect, their specific refrac- 

 tive energies being respectively 0*081 and O'Oll. Again it will be 

 evident that the proportion between the refraction and dispersion is 

 not the same even in the case of analogous elements. Thus, taking 

 the three halogens, the ratio between the refraction for A and the 

 dispersion for H— A for chlorine is about 100 to 5, for bromine 100 to 

 8, and for iodine 100 to 15. 



Metals in Salts. — In 1869, as already stated, I suggested that the 

 same data from which the refraction equivalents of the metals had 

 been determined, would be available also for their dispersion equiva- 

 lents. I have many observations in addition to the data then 

 published ; and Kanonnikoff has been over part of the same ground, 

 measuring the a and ft of the hydrogen spectrum. Unfortunately, 

 however, the errors of observation bear so considerable a proportion 

 to the whole amount observed, at any rate in dilute solutions, that 

 we cannot look upon single determinations of the dispersion equiva- 

 lent of a salt as of much value. Thus, even when great care has been 

 taken in measurement, each index of refraction is liable to an error of 

 ±0*0001, and as the error in determining A and H may be in 

 opposite directions, /u> K — /* A cannot be relied upon within +0*0002. 

 Now among solutions of salts the specific dispersion rarely amounts to 

 0*02 ; the error of observation may therefore be more than 1 per cent., 

 and if the salt should form only 5 per cent, of the solution, the error 

 might exceed 20 per cent. Such solutions, therefore, are practically 

 valueless for this purpose. Yet it would be easy to publish a table of 

 miscellaneous salts, the dispersion equivalents of which had been 

 deduced from several fairly accordant observations on fairly strong 

 solutions, or which have been corroborated from some independent 

 source. It has appeared preferable, however, to confine attention at 

 present to the series of potassium and sodium salts, which are far the 

 most complete and the most instructive. 



It is evident at a glance, that the figures in the sodium columns are 

 invariably lower than those in the potassium columns, and that the 

 difference is fairly uniform. In regard to the refraction equivalent, 

 it is about 3*33,* and in the dispersion equivalent it is about 09. 



It follows, that if we can determine the value of potassium, that of 

 sodium may be at once calculated : and presumably the same process 

 may be extended to all other metals that form soluble salts. 



But it is not so easy to determine the value of potassium. In 



* Previously determined at 3*3. 



