86 



Prof. H. Bunsen on Flame Reactions. 



are altered if the temperature of evaporation and the weight 

 of the substance are changed. It is, however, remarkable that 

 an approximate relation appears to hold good, probably a mere 

 accident, between the atomic weights and the volatility of those 

 analogous bodies which are easily evaporated. The following 

 numbers show that the volatility of those haloid compounds 

 which evaporate without residue is directly proportional to their 

 atomic weights : thus if A signify the atomic weight, and F the 



F 



volatility, we find that the equation -r- = const, is nearly true. 



F. 



Lithium-chloride . 



. 4249 



0-739 



A 

 0-0174 



Sodium-chloride 



. 58-43 



1-000 



0-0171 



Potassium-chloride . 



. 74-57 



1-288 



00173 



Sodium-bromide . 



. 102-97 



1-727 



0-0168 



Potassium-bromide 



. 119-11 



2-055 



0-0173 



Rubidium-chloride . 



. 120-82 



2-183 



0-0181 



Sodium-iodide . . 



. 15007 



2-360 



0-0157 



Csesium-chloride . 



. 168-46 



2-717 



0-0161 



Potassium-iodide . 



. 166-21 



2-828 



00170 



Drops of water, alcohol, ether, and other volatile liquids 

 brought into the flame on the platinum wire do not boil even in 

 the hottest part of the flame, and hence they exhibit under these 

 circumstances the phenomenon of the spheroidal state. 



4. Flame-coloration. — Many substances which volatilize in the 

 flame may be detected by the peculiar kinds of light which their 



flowing gases emit. These colorations appear in the upper oxi- 

 izing flame when the substance causing them is placed in the 

 upper reducing flame. Mixtures of various flame-colouring sub- 

 stances are tested in the lowest and coldest part of the flame ; and 

 here it is often possible to obtain for a few moments the peculiar 

 luminosity of the most volatile of the substances unaccompanied 

 by that of the less volatile constituents. 



B. Oxidation and Reduction of Substances. 



In order to recognize substances by the phenomena exhibited 

 in their oxidation and reduction, and to obtain them in a fit 

 state for further examination, the following methods are em- 

 ployed : — 



1. Reduction in glass tubes is especially employed for the de- 

 tection of mercury, or for the separation of sulphur, selenium, 

 phosphorus, &c. when in combination with sodium or magne- 

 sium. A stock of very thin glass tubes is prepared, each 2 to 4 

 millims. in width and 3 centims. in length : forty of these are 



