338 PROCEEDINGS OF THE AMERICAN ACADEMY 



Professor Levering suggested whether the nodal lines of 

 acoustic vibration had anything to do with the phenomenon. 



Professor Peirce made an elaborate communication on the 

 tides. 



Professor W. B. Rogers gave an account of some experi- 

 ments which he had been making on the phenomena of sub- 

 jective vision, to settle the question whether the two eyes 

 combined successively the different points of an object, as 

 Brewster supposed, or obtained an instantaneous recognition 

 of all parts at the same moment. 



Mr. F. H. Storer read by title a " Memoir on the Alloys of 

 Copper and Zinc." 



He also presented the following communication : — 



On the Detection of Chrotnium in Presence of Iron. 



It is customary, in the methods of analysis now most commonly em- 

 ployed in laboratories of instruction, to rely upon the solvent action 

 which the caustic alkalies exert upon hydrated sesquioxide of chromi- 

 um as a means of separating this base from the oxides closely allied to 

 it. It is well known, however, and the experiments of Northcote and 

 Church * have determined the fact quantitatively, that when a small 

 amount of sesquioxide of chromium is accompanied by a large quantity 

 of the oxides of manganese, cobalt, nickel, or of sesquioxide of iron, it 

 ceases to be soluble in the alkalies. From the frequency of its occur- 

 rence, as well as from the fact that it has the power of concealing a 

 larger amount of chromium than either of the other bases which have 

 been mentioned, the sesquioxide of iron in particular gives rise to 

 much inconvenience in practice. It is a constant source of annoyance 

 to beginners, who almost invariably fail to detect the presence of chro- 

 mium in solutions containing it given them for analysis, if these at the 

 same time contain iron also. In this case it may be detected, it is true, 

 by fusing the mixed precipitate of oxide of iron and of chromium 

 with nitrate of potash and carbonate of soda, and examining the aque- 

 ous solution of the mass obtained for chromic acid ; but the student 

 seldom applies this test, unless specially directed to do so. The oper- 

 ation is troublesome, since it necessitates the employment of a special 



* Qu. J. Chem. Soc , VI. 53. 



