Prof. R. Bunscn on Flame Reactions, 105 



any film on porcelain in the upper oxidizing flame, the phos- 

 phates may be recognized by heating on platinum with borax 

 and a thin piece of iron wire in the hottest part of the reducing 

 flame, when a bright molten bead of iron-phosphide is obtained 

 which can be extracted with the magnetized knife on crushing 

 the bead under paper. 



35. Sulphur Compounds. 



(a) These give a fused mass with soda on the charcoal splin- 

 ter, which leaves a black stain when brought on to moistened 

 silver. As selenium and tellurium give rise to the same reaction, 

 the absence of these substances must first be ascertained by 

 seeing that no film of selenium or tellurium is deposited on 

 porcelain. 



(b) In the case of metallic sulphides, and not of sulphates, 

 the simple ignition of the sample in the flame is sufficient to 

 produce the peculiar smell of burning sulphur. 



It is impossible in a limited space to follow out the applica- 

 tion of the foregoing methods to all the special cases of the ana- 

 lysis of complicated mixtures. I confine myself, therefore, to 

 illustrating the value of those methods in one or two examples. 



1. A mixture of the sulphides of arsenic, antimony, and tin. 



If in a mixture of these three sulphides, containing only traces 

 of antimony and tin, they are separated according to the ordinary 

 rules of qualitative analysis, by dissolving in alkaline sulphides and 

 reprecipitation with acids, the detection of these two metals by 

 the regular tests is extremely uncertain and troublesome. Ac- 

 cording to the following method, the detection of these metals is 

 rendered easy and certain when the proportion of tin is only a 

 few thousandths, and that of the antimony only a few hundredths 

 of the total weight of the mixture. 



Three decigrammes of the sulphides are roasted on a curved 

 piece of glass, small enough to be altogether surrounded by the 

 flame, and the residue, weighing only a few milligrammes, is 

 scraped together with the knife. The moistened mass is then col- 

 lected on the end of a thread of asbestos, and a strong metallic film 

 obtained on the test-tube (see page 90). In order to prevent 

 the deposition of any carbon with the metals, which would act 

 injuriously in the subsequent operations, the upper reducing 

 flame is made so small that the luminous point is only just visi- 

 ble. The film is next dissolved in a drop or two of nitric acid 

 in the curved rim of the plate (fig. 3), and the solution evapo- 

 rated below its boiling-point by gently warming and blowing, so 

 as to obtain the solid residue in as small a space as possible. 



