412 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



PRELIMINARY COMPARISON. 



According to these preliminary results the varying conditions seemed 

 to affect the weight of silver by only one part in ten thousand, the 

 higher temperature yielding this slight excess of weight. 



It became now a matter of interest to determine if this slight differ- 

 ence might be yet further reduced by the employment of yet greater 

 care. Accordingly three much more accurate experiments were made, 

 in a large dark-room which was for the time devoted wholly to this work. 

 No injurious gases were allowed in the vicinity, and dust was as much 

 as possible excluded. The current of 0.2 ampere was open for nearly 

 two hours in each experiment. The impurity included in the crystals 

 was more carefully determined in every case, the deposit from the 

 aqueous solution being ignited at a dull red heat, while the alkali nitrate 

 residues were converted wholly into sulphate, and freed from a last trace 

 of silver by means of hydrochloric acid. The final dried residues of 

 mixed sulphates weighed respectively 0.30, 0.54, and 0.32 milligrams. 

 Since the nitrates of sodium and potassium were present in molec- 

 ular proportions, and since the hot mother liquor contained ten per 

 cent of argentic nitrate, these values correspond to corrections of 0.39, 

 0.69, and 0.42 milligrams respectively — corrections which are applied 

 below. 



The mother liquor included in the silver formed in the cool cell must 

 also have contained dissolved argentic nitrate to the extent of about 

 0.02 milligrams in each case, an unimportant amount which is neverthe- 

 less subtracted below for the sake of completeness. 



All weighings were made by substitution on a very precise balance 

 with weights carefully corrected ; and in general all the precautions 

 needed in work upon atomic weights were observed. 



The final figures thus obtained are as follows : — 



