OBSERVATIONS IN CHEMISTRY AND MINI RAUXn \W 



center of the drop by means of a suitable pair of forceps. Now bring the rim 

 of the persulfate into focus and watch events. Bubbles of oxygen appear first 

 then a green fringe appea s if the percentage of cobalt is not too small. A few 

 seconds later the microlites of the praseosalt begin to float across the field f 



vision, owing to the currents produced by the escaping oxygen-gas. When I 

 saw this phenomenon first, the concentration happened to be ju t right for the 

 forming of the type of crystals which I call "bird on wing or butterfly tvpe," 

 and I was fascinated, for the thing- seemed to be animated, ener? 'ticallv starting 

 upon the best exploitation of their ephemeral course. If no greet microliter 

 come up within two minutes the absence of cobalt is by no means proven. Such 

 proof is brought only by the absence of crystals after the liquid has dried up. 

 We see first, shooting from the rim toward the center, the colorless prisma ti< 

 crystals of ammonium sulfate. In them are found imbedded the simple prisms 

 of the praseosalt with a certain likeness to some formsof stained illi." Not 



only the number of crystals but also their dimensions are decreasing with incr< 

 ing dilution; but not without exceptions, owing, no doubt, to other attending 

 or surrounding conditions. In a few cases larp' and well-formed ci -tals 

 were obtained, when all present trace of material went into tli forming of one 

 crystal, instead of splitting up into many tii y ones. Some mineral localities 

 furnish analogous facts, when the rarest elements are fotind in a few very larjpe 

 crystals. One remembers the enormous crystals of beryl, nionazitc, microlit 

 at Amelia C. H., Va. In watching all of the phases in the drying up of a drop, 

 one sees at the end a quick shooting of a kind of veil across the field; it is amm« 

 nium chloride and ammonium nitrate. This is not yet complete drynesi and I 

 have seen praseo crystals grow even then. But to see them one must remove 

 the overlying veil. This can be done with a spray of ammonia water. The 

 field clears up and must then be carefully looked over, meaning not onl; the 

 field just in vision but the entire surface of the drop, which is usually from 



14 to X A of an inch in diameter. 



< 



Since all the minerals which carry cobalt and nickel are deeomix^ d by a<jiia 

 regia, they may be subjected to the test for cobalt by this microscopic test, 

 requiring only a very small trace of substance, very much less than is n< ded f<»r 

 the Plattner arseniation method, which, of course, is in itself a most excellent one 

 It is best to eliminate the arsenic by careful roasting of the mineral powder upoi 

 a surface of charcoal until the odor of arsenic is not perceptible, because e pcrience 

 shows that the presence of arsenic acid in the ammoniacal solution may prevent 

 the praseo reaction, physically through the forming of a white fog or veil, chemic- 

 ally through the precipitation of nickel and cobalt together as whi ar eniate 

 of indefinite forms. Sometimes it may happen that in presence of much copper 

 the praseosalt falls out in such very small crystals that one fails to recognize 

 them. In such cases allow the drop on slide to dry, then put one drop of 

 concentrated ammonia water upon it. The minute microliter then dissolve, 

 while the large copper-salt crystals remain intact, and as evaporation now proceeds 



