342 M. Buzengeiger on the Colours ichich different 



common lamp-oil. Oil that has been purified by sulphuric 

 acid ought to be rejected for experiments with the blowpipe, as 

 it always retains a little of the acid, and then deteriorates the 

 wick, and changes the colour of the external vapour. This oil 

 is immediately known, because one is obliged to cut the wick 

 frequently, on account of its becoming transformed into a dirty 

 matter, presenting the reaction of sulphur. The wick ought to 

 be formed of raw cotton thread, because the bleaching is often 

 performed with chloride of lime, which communicates a reddish- 

 yellow tint to the external vapour, which must be avoided, as 

 the blue colour is necessary to the success of the experiments. 



As to the manner of blowing, it is necessary to acquire the 

 power of keeping up the blue conical flame well developed and of 

 constant length, without the yellow flame becoming mixed with 

 it. The external blue vapour is then perceived, at least when 

 the daylight is not too strong. The experiments should there- 

 fore be made in the evening in a dark room. 



To make the experiments, the assay piece is seized with a 

 platina forceps ; and, when the flame is in a suitable state, it is 

 introduced from beneath upwards, within the external vapour, 

 before the blue point. The form of the assay piece depends 

 upon particular circumstances. It may be in bits of various 

 sizes, wedge-shaped, acicular, or in small laminae. 



It is often necessary to pulverize it. It is then made up in- 

 to paste in the hollow of the hand, spread out upon a bit of 

 charcoal, properly shaped ; and, lastly, heated until it acquires 

 sufficient consistence to be held with the platina forceps. 



The following phenomenon is what usually takes place. 

 When the assay piece is immersed in the blue vapour before 

 the blue cone, the former is immediately replaced by a red- 

 dish-yellow atmosphere, the extent and intensity of which de- 

 pend upon the nature of the body submitted to trial. By de- 

 grees the atmosphere diminishes and disappears. Another phe- 

 nomenon is then produced : the blue vapour bathes the body 

 without being altered, and remaining hardly visible, or it as- 

 sumes a colour which varies with the nature of the body which 

 is volatilized at this stage of the experiment. 



Only three substances are yet known which present the red co- 

 1 



