RICHARDS AND PARKER. — ATOMIC WEIGHT OF MAGNESIUM. 57 



was formed, ami he does not appear at all confident of the accuracy of 

 his results. From the experience of the writers it does not seem likely 

 that the method which he used would give magnesic chloride free from 

 the oxide. 



Prellminary Experiments. 



Because considerable experience had been gained in a previous re- 

 search.* upon the occlusion by baric sulphate of salts present in a solution 

 from which this insoluble salt was precipitated, it was thought that Gay- 

 Lussac's and Scheerer's method of precipitating magnesic sulphate with 

 baric chloride might now be used with advantage, applying subsequently 

 the necessary corrections for occluded substances. It had previously 

 been found that the concentration of the solution and the method of 

 pouring had a great deal to do with the amount of occlusion ; and hence 

 it seemed likely that, by working in a very dilute solution and pouring 

 the magnesic sulphate into the baric chloride with extreme slowness, the 

 occlusion of baric chloride might be lai-ge, but that the precipitate might 

 be free from magnesium. Several experiments were made to ascertain 

 the correctness of this supposition, but in each case it was found that, 

 notwithstanding the precautions adopted, a very notable quantity of 

 magnesium was occluded in the baiic sulphate. It had been the custom 

 in working upon this precipitation to fuse the weighed baric sulphate with 

 sodic carbonate, to extract the sodic chloride thus formed, and to deter- 

 mine the chlorine with argentic nitrate and calculate as baric chloride, 

 subtracting this amount from tlie total weight of baric sulphate found. 

 This method gave very satisfactory results, but of course it could not be 

 applied when the baric sulphate was mixed with magnesic chloride and 

 sulphate as well as baric chloride, for then no one could discover the 

 proportion in which each salt was present, with sufficient accuracy for 

 work upon atomic weights. 



The possibility of obtaining .satisfactory resi;lts by the determination 

 of the chlorine in magnesic chloride was now considered. The great 

 disadvantage of this method, as is well known, is the extreme difficulty 

 of obtaining pure anhydrous magnesic chloride. The usual method of 

 iiTiiiting the double chloride of ammonium and magnesium was tried a 

 number of times, but it was found that a quantity of the ox3'chloride was 

 always formed. As indicators do not give a sharp reaction in the pres- 

 ence of magnesic salts, the hydrochloric acid driven off cannot be added 



* These Proceedings, Vol. XXXI. p. 67 



