Prof. R. Bunsen on Flame Reactions. 



85 



tronome, whicli the bead takes to volatilize. The point at which 

 the whole of the substance is converted into vapour can be ascer- 

 tained with great accuracy, often to a fraction of a second, by the 

 sudden disappearance of the coloration of the flame. The plati- 

 num wire upon which the substance is weighed is protected from 

 the moisture of the air by insertion in a tube (fig. 7). If we 

 know the weight of the tube and wire, the right weight of sub- 

 stance can easily he attached, either by volatilizing a portion or 

 by fusing some more substance on to the bead, and thus making 

 it lighter or heavier. The experiments are best made with one 

 centigramme of substance. The position in the flame where the 

 highest constant temperature exists can be found by moving a 

 fine platinum wire, fixed on a stand and bent at its point at a 

 right angle, slowly about the zone of fusion, and noting the point 

 where it glows most intensely. The beads to be volatilized are 

 then most carefully brought into the flame at the same distance 

 from the point of this wire. Care must also be taken that the 

 dimensions of the flame do not undergo change from alterations 

 in the pressure of the gas whilst the experiments are going on. 

 As a measure of volatility, it is convenient to take the time 

 needed to evaporate one centigramme of the sodium-chloride. 

 If we call this time t , and the time of volatilization of the same 

 weight of another substance t v the volatility of this substance, 

 compared with that of common salt, is 



f='f 



l \ 



The following examples are taken from experiments of Dr. 

 Hurtzig, to which I have added the chlorides of csesrum and ru- 

 bidium : — 



Tin 



le of evaporation. 



Volatility. 



Sodium-chloride 



84-25 



1-000 



Sodium-sulphate . . 



. 1267-0 



0-066 



Lithium-carbonate 



736-5 



0-124 



Potassium-sulphate 



. 665-2 



0-127 



Sodium-carbonate . 



. 632-0 



0133 



Potassium-carbonate . 



. 272-0 



0-310 



Lithium-chloride . 



114-0 



0-739 



Potassium-chloride 



65-4 



1-288 



Sodium-chloride 



48-8 



1-727 



Potassium-bromide 



41-0 



2-055 



Rubidium-chloride . . 



38-6 



2-183 



Sodium-iodide . . . 



35-7 



2-360 



Caesium-chloride . . . 



31-3 



2-717 



Potassium-iodide 



29-8 



2-828 



It must not be forgotten that the relations of these numbers 



