88 Prof. R. Bunsen on Flame Reactions. 



to a small piece of curved glass cut out from an old flask*, in 

 which they are again washed by decantation, the last drops of 

 water removed by suction with a piece of filter- paper, and the 

 metallic particles dried at a moderate heat. A few tenths of a 

 milligramme of the metal is generally sufficient to yield a solu- 

 tion with which all the characteristic precipitations can be accom- 

 plished, the reagents being contained in capillary glass threads, 

 dropped into the solution by the milligramme, and the effect thus 

 produced ascertained by examination with a lens. Iron, cobalt, 

 and nickel, which do not fuse to globules on the splinter, are 

 withdrawn from the agate mortar by means of the point of the 

 magnetized blade (fig. 4, b), washed with water, and dried high 

 above the flame on the point of the knife. If the blade be then 

 tightly drawn between the upper part of the thumb and the lower 

 part of the first finger, and if the point of the blade be then ap- 

 proached to the metallic particles on the finger, they jump from 

 the hand to the blade, forming a brush-like bundle which can be 

 conveniently examined by the lens, and by touching with a melted 

 borax-bead can be transferred in suitable quantities. The por- 

 tion of metal remaining on the knife is rubbed on to a small 

 piece of filter-paper, a drop of acid added, and the paper warmed 

 over the flame so as to allow the metal to dissolve ; this solution 

 can then be further examined with various reagents. 



3. Films upon Porcelain. — Those volatile elements which are 

 reduced by carbon and hydrogen can be deposited from their 

 compounds as films on porcelain either in the elementary state 

 or as oxides. Such films can be extremely easily converted into 

 iodides, sulphides, and other compounds, and thus may be made 

 to serve as most valuable and characteristic tests. The films are 

 composed in the centre of a thicker layer, which on all sides gra- 

 dually becomes thinner until the merest tinge is reached; it is 

 therefore necessary to distinguish between "thick" and "thin" 

 parts of the films. Both exhibit in their variation of thickness all 

 the tints of colour characteristic of the substance under different 

 circumstances of division. One-tenth up to one milligramme 

 is in many cases sufficient for these reactions. Many surpass 

 Marsh's arsenic test in delicacy and certainty, and approach 

 in this respect the spectrum-analytical methods. 



The following films can be obtained : — 



(a) Metallic films are prepared by holding in one hand a par- 

 ticle of the substance on an asbestos-thread in the upper reducing 

 flame, which must not be too large, whilst with the other hand 

 a glazed porcelain basin, 1 to 2 decimetres in diameter, filled 

 with cold water, is held close above the asbestos-thread in the 

 upper reducing flame. The metals separate out as dead-black 

 * Watch-glasses crack much too readily to be used for such experiments. 



