RICHARDS. — ATOMIC WEIGHT OP BARIUM. 87 



amounts of hydrochloric acid were therefore added very gradually to 

 the main mass of the liquid, until, after agitation and settling, two por- 

 tions of the clear supernatant liquid treated respectively with argentic 

 nitrate and hydrochloric acid showed like intensity of opalescence. 

 This similarity of opalescence was further confirmed by measuring 

 the heights of parallel unknown columns of the two liquids which had 

 been adjudged equally cloudy. From the average of a great number 

 of readings, one could feel reasonably certain of the result within two 

 or three per cent, corresponding to an error of not over one or two 

 tenths of a milligram of silver in the volume of solution used. 



For example, in the first analysis recorded below, it was found in 

 three preliminary trials that about 0.30 cubic centimeter of the stand- 

 ard hydrochloric acid should be added to the remaining mixture in 

 order to reach the desired point. This amount was therefore added, 

 and two portions were again taken out, treated as before, and compared. 



This average shows that the equality of cloudiness was now within 

 the probable experimental error. 



It was borne in mind, however, that a fifth of the original solution 

 had been used up in the preliminary tests. Hence, the true amount 

 of hydrochloric acid which should have been added to the original 

 solution must be £ X 0.3 = 0.375, or practically 0.4 cubic centi- 

 meter. 



Hence, since this amount of hydrochloric acid is of course a direct 

 measure of the excess of silver originally taken, 0.4 milligram should 

 be subtracted from the original weight. 



It is needless to say that all possible care was used in these final 

 determinations. The reiteration of the countless details would be 

 wearisome, and hence these details are left to be inferred from what 

 has been already written. The method is the simplest and easiest of 

 the four ; and when one has acquired experience in the comparison 

 of opalescence, it is perhaps also the most accurate. 



