204 E. H. Jenkins — Effect of Silicic Acid upon the 



cubic planes, and also the form 3-3. It has a jellowish-brown 

 color and resinous luster. Professor Brush reports, from his 

 examination, that in blowpipe characters it agrees closely with 

 pyrochlore ; but its specific gravity as determined by him on a 

 pure crystal is 4-794, which is considerably higher than that of 

 pyrochlore (4-203, Hermann), so that it may approach more 

 nearly to microlite. For a definite knowledge of its character 

 we must consequently wait for the chemical analysis which 

 Professor Allen proposes soon to undertake. These octahe- 

 drons occur generally in a rusty gangue, the mass of which 

 seems to consist mostly of the same mineral. They are also 

 sometimes observed implanted directly upon the samarskite. 



The second associated mineral is columhite. It occurs in crystal- 

 line masses of considerable size, imbedded in the samarskite, or 

 implanted upon it The form where distinct is very similar to 

 those given in Dana's Mineralogy, figures 429, 430, p. 516, and 

 the angles agree closely. From some qualitative experiments 

 Professor Allen finds that it contains a considerable quantity of 

 tantalic acid. On this account it is a matter of some surprise 

 that its specific gravity is only 5*476. 



This intimate association of columbite and samarskite at this 

 locality is the more interesting in that, as long ago shown by 

 Hermann, these two species occur together at Miask in the Urals. 

 Some Uralian specimens recently examined by me have the 

 minute crystals of columbite, well formed, implanted on the 

 samarskite, the crystals of the two appearing to occupy a parallel 

 position. It would here hardly be suspected that the two min- 

 erals were distinct, except from the cross fracture, in which the 

 two decidedly differ. The American specimens, on the other 

 hand, with the single exception alluded to, show no relation at 

 " I the position of the crystals of the 1 



Art. XXVIL— TAe Effect of Silicic Acid upon the Estimation of 

 Phosphoric Acid hy Ammonium Molyhdate ; by E. H. Jenkins. 



The idea seems to be general that the presence of silicic 

 acid in solutions, impairs the accuracy of the estimation of 

 phosphoric acid by the molybdic method. In Rose's Hand- 

 buch der Analytischen Chemie, 6th edition, volume ii, page 

 519, under a description of this method the fact is stated 

 that silicic acid gives a precipitate similar to the ammonium 



