Law of Density ^Number >$. 



21 



In the following pages I shall endeavour to give a fairly 

 complete account of the numerous applications of this law, 

 the study of which has occupied my leisure hours for many 

 years, ever since a happy chance disclosed to me a special 

 case, an account of which was published in Poggendorff's 

 Annalen, lxxviii. (1849) p. 112. 



When the law is applied to hydrated crystalline salts con- 

 taining the same number of molecules of water, a hitherto 

 concealed relationship is discovered. 



If B is the sum of the numbers of density of the salt, and 



8 its density, it is found that ^ =k for all salts with similar 



formula and containing the same amount of water of crystal- 

 lization, as will be seen from Table II., which contains the 

 data relating to six salts of the general formula MR 2 . 6H 2 0. 

 B in this case is equal to the sum of the density-numbers for 

 the anhydrous salt plus 18, which is the value of B or n for 

 H 2 0. Thus, a reference to Table I. shows that CaCl 2 6H 2 

 has B = 7 + 2(4) + 6(3) = 33. 





Table IL 





Salt. 



B. 



S. 



Jc. 



SrCl 2 , 6aq 



21 + 18=39 

 15 + 18=33 

 19 + 18=37 

 13+18=31 

 23+18=41 

 23+18=41 



1-964 



1-654 



1-84 



1-562 



205 



2-065 



19-86 

 19-95 

 20-11 

 19-85 

 20-00 

 19-85 



CaCl 2 ,; 



CoCl 2 „ 



MgCl 2 „ 



Ni(N0 8 ) a> 6aq.... 



Zn(N0 3 ) 2 „ ... 



The data for this table and for others are taken from the 

 collections made by Landolt and Bornstein, Clarke's c Constants 

 of Nature,' and the experiments of Bodeker, Playfair and 

 Joule, Schroder, Topsoe, and others on the densities of com- 

 pounds. 



The table shows clearly enough that the volumes k are 

 practically equal, and by means of such groups it is possible 

 to determine the density-numbers for the various elements. 

 All the density-numbers in Table I. have been determined by 

 employing the data given by hydrated salts and (where pos- 

 sible) the boiling-points, and the densities of aqueous solutions 

 as well as all other available analogous data. 



At this point I shall return to Table I., in order to bring 

 forward some views of a more general nature regarding the 

 density-numbers. 



