442 



Dr. Gladstone on Refraction- Equivalents, [June 18. 



it is evident that the refraction-equivalents of the compounds of potassium 

 differ very widely according to the nature of the electro-negative con- 

 stituents ; again, that the refraction-equivalents of the compounds of 

 sodium differ pari pasm with those of the potassium compounds, being 

 always less by a number varying from 3*0 to 3*9. From this it may be 

 fairly concluded that the electro-negative constituent has the same effect on 

 light, whichever metal it is united with, and that the refraction-equivalent 

 of potassium exceeds that of sodium by 3"4, or thereabouts. But does the 

 Table afford the data for determining the absolute equivalent of one or 

 other of these metals ? It was at first thought that this would be arrived 

 at by a comparison of the metal with hydrogen, the refraction- equivalent of 

 which has hitherto been estimated at TS (or 1*5 in the case of water) ; but 

 the last column shows that the difference between potassium and hydrogen 

 is not always the same, the differences being greater than can be attributed 

 to errors of observation. Indeed the numbers seem to fall into two 

 groups : with the mineral acids the differences lie between 4*0 and 4*7, 

 while with water, alcohol, and the organic acids, they are always upwards 

 of 6, varying indeed from 6 '2 to 6 '8, the average being 6*55. But it is in 

 these last-mentioned compounds that the equivalent of hydrogen is believed 

 to be 1*3. Assuming this, we may reckon the refraction-equivalent of 

 potassium to be about 6-55 + 1*3, that is, 7'85. We have, however, 

 other means of arriving at an estimation. Chlorine, in such bodies as 

 chloroform or tetrachloride of carbon, is represented by 9 "8. Again, 

 cyanogen, from the experiments of Dulong on the gas itself, maybe taken at 

 9'2 Sulphur has a refraction of 16"0; hence sulpliocyanogen may be 

 reckoned as 16*0 + 9*2, that is, 25"2. Subtracting these numbers from 

 those of the respective potassium salts, we obtain the equivalent of the 

 metal. Thus from different sources we may calculate for the value of 

 potassium : — 



From the chloride 8*6 



„ cyanide 7" 9 



„ sulphocyanide 8*2 



„ hydrate...... 8*3 



„ alcoholate . . . . . . 8*1 



,, formiate 7*8 



„ acetate 7*7 



„ tartrate , 7' 5 



These numbers are tolerably close, though the equivalent of potassium, 

 deduced from its inorganic, would, on the whole, be higher than that 

 deduced from its organic compounds. The mean of the first four compu- 

 tations is 8*2, that of the last four 7"8. Perhaps, pending further re- 

 searches, it will be best to assume the mean of these numbers. 



Potassium 



= 8-0; 



