1868.] 



Dr. Gladstone on Refraction-Equivalents. 



443 



and since a sodium salt has a refraction-equivalent generally 3*4 lower 

 than the corresponding potassium salt, we may reckon 



Sodium =4-6*. 



If instead of taking the refraction-equivalent P ^ ^ ^ ^ we reckon the 



specific refractive energy ^ ^ , we obtain the following values : — 



Potassium 0*205 



Sodium 0-200 



This implies that equal quantities of these two analogous metals exert 

 very nearly, if not precisely, the same effect on the velocity of the rays of 

 light. 



Another deduction from the above Table is that already alluded to in 

 regard to hydrogen ; while in the organic acids it probably has the known 

 refraction-equivalent 1'3, it would seem that in the others, viz. hydro- 

 chloric, hydrobromic, hydriodic, nitric, metaphosphoric, and sulphuric 

 acidsj it has a very much higher refraction-equivalent, one in fact which is 

 little less than sodium, and falls short of potassium by only about 4*3. 

 Hence we may we deduce — 



Hydrogen in organic compounds .... =1*3 

 Hydrogen in mineral acids =3*7 



How far this conclusion may hold good throughout, and whether one 

 number should be an exact multiple of the other number, must remain to 

 be determined by future observations. It appears, however, to answer in 

 the affirmative the question whether an element can have more than one 

 definite refraction-equivalent. 



Of course, from the Table given above, it would be easy to deduce 

 values for each of the electro-negative constituents ; but it would be safer 

 to generalize from a larger number of instances. 



The series of observations on potasisum and sodium salts are the most 

 complete and the most carefully revised of any which have yet been made ; 

 but if their refraction-equivalents are fixed, it becomes a much simpler 

 matter to determine those of most other metals. Thus, of lead the nitrate 

 and acetate have alone been examined ; but as the refraction-equivalents 

 reckoned for these salts differ from those of the corresponding potassium 

 compounds in each instance by 4*1, the presumption is great that the 

 refraction-equivalent of lead is very near 8*0 -|- 4" 1, or 12*1. 



The following are numbers deduced from two or more salts of each 

 metal. They must be looked upon only as approximately true, and any 

 subsequent modification of the value 8'0 for potassium, must lead to a 

 corresponding modification of the whole series. 



Haagen, from the crystalline chloride, determined the refraction -equivalent of 

 sodium for the hydrogen line d at 4*89. 



