30 



MACGEEGOE ON DENSITY AND 



The following are other observations with the same substance : — 



Thomseu's experiments shew that in the case of solutions of Caustic Potash, even up 

 to a strength of more than 5 per cent., excess of density over that of vpater is practically- 

 proportional to the percentage of salt. Kohlrausch's resiilts substantiate Thomsen's ; for, 

 as is seen above, a formula in which k is chosen so that the density given by it is exact 

 for^ = 4' 19, gives a density for^ =: 8"42, which not very far wrong. 



Caustic Soda. — NaHO. 



Observer : Thomsen. ^ 



Temperature : 18°C. 



Formula : Di^ = • 99866 + 0-014563 p. 



The formula therefore holds for solutions of strengths ranging up to about 2 per 

 cent., but not for solutions of greater strength. Caustic Soda thus differs in a marked 

 manner from Caustic Potash. 



The above are the only salts for which I have been able to obtain, or to find, data to 

 determine the limits of concentration^within which formulœ of the above kind hold — 

 within which, in other words, the concentration-density curves of their solutions are, to 

 the fourth place of decimals, straight lines. In the case of most salts, the weakest 

 solutions, whose densities have been examined, are already beyond the limits referred to. 



The table upon the page that follows contains a list of the values of k found above 

 for the various salts examined. 



It will be noticed that in most cases the values of k do not differ from one another to 

 any considerable extent. Now k is the rate of increase of the density with the strength 

 of a solution when its strength is but small. Hence the densities of dilute solutions of 



' AVied. Ann. vi (1879) p. 21. 



■^ Thermo-chemische Untersuchungen, Bd. i, p. 47. 



