120 Dr. J. J. Hood on the 



then, by fractional decomposition of the nitrates by heat or by 

 fractional precipitation, the tendency of one constituent to be 

 decomposed or precipitated before another will be so much 

 the greater as the difference in their basicities is greater, and 

 their separation by either process consequently so much the 

 easier. 



Supposing, however, that two such oxides were of the same 

 basic strength in relation to all acids and precipitants, and at 

 all temperatures at which the experiments could be conducted, 

 then it is evident that in such a case no separation could be 

 effected, however many times the operations were repeated on 

 the fractions obtained. If the ratios of the basic strengths 

 vary with the temperature, which is probable, and with the 

 nature of the precipitant, which may not be true, then it is 

 quite possible that, in the separation of some of the rare earths 

 by fractional precipitation, the temperature at which the 

 experiments are made and the precipitant employed may be 

 the very worst possible that could be selected. 



From a theoretical point of view, the problems presented by 

 fractional precipitation would seem to be of considerable im- 

 portance, and the process offers a wide and doubtless a fertile 

 field for experimental study. Viewed in a practical light, or 

 as regards the application of the process to the separation of 

 bodies differing in basic strength, the problem to be solved is: 

 Given a mixture of two or more such substances, what is the 

 best method of conducting the operations so that a given mass 

 of material may yield a maximum of one or other of the pure 

 oxides with the least amount of labour or the smallest number 

 of precipitations ? Evidently, for an answer to this question 

 to be of value, the theoretical side of the problem must first 

 be studied experimentally for the commoner elements to 

 decide whether or no the ratios of the basic strengths of the 

 materials vary with the temperature and in what manner, 

 decreasing or increasing as the temperature is increased ; 

 whether these ratios vary with the nature of the precipitant 

 or of the acid combined with the base, and whether retarding 

 the precipitation b}^ introducing extraneous bodies into the 

 chemical system affects the ratio of the materials in the pre- 

 cipitate. In other words, under what conditions can the ratio 

 of the less basic to the more basic material in the precipitate 

 be made the greatest possible by adding a fraction of the total 

 precipitant to a solution containing two salts ? 



In such a chemical change as is expressed by the equation 

 NiS0 4 + Na 2 C03 = NiC03 + Na 2 S0 4 , if the chemical affinities 

 operating among the constituents be considered, it is evident 

 that, however simple the final result of the change may be 



