314 Royal Society : — 



termining 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 1*3 (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 attri- 

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

 two groups : with the mineral acids the differences lie between 4 - 

 and 4*7, while with water, alcohol, and the organic acids, they are 

 always upwards of 6, varying indeed from 6*2 to G'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 arri- 

 ving 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, may be taken at 9*2. Sul- 

 phur has a refraction of 16'0 ; hence sulphocyanogen may be reck- 

 oned as 16-0 + 92, 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 po- 

 tassium deduced from its inorganic, would, on the whole, be higher 

 than that deduced from its organic compounds. The mean of the 

 first four computations is 8*2, that of the last four 7*8. Perhaps, 

 pending further researches, it will be best to assume the mean of 

 these numbers, 



Potassium — 8'0 ; 



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 



Cu 



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



Potassium 0*205 



Sodium 0-200 



This implies that equal quantities of these two analogous metals 



* Haagen, from the crystalline chloride, determined the refraction-equiva- 

 lent of sodium for the hydrogen line d at 4*89. 



