84 Prof. R. Bun sen on Flam,e Reactions. 



perature. The following six different temperatures can be ob- 

 tained in the flame, and these points may be judged of by obser- 

 ving the tints attained by the thin platinum wire : — 



1. Below a red heat. 



2. Commencing red heat. 



3. Red heat. 



4. Commencing white heat. 



5. White heat. 



6. Strong white heat. 



It is scarcely necessary to remark that these different tempe- 

 ratures must not be ascertained by the glow of the substances 

 themselves, as the luminosity of different bodies depends not 

 only upon the temperature, but also mainly upon their specific 

 power of emission. 



The following phenomena are observed when a sample of a sub- 

 stance is heated:- — 



1. Emission of Light. — The emissive power of substances is 

 ascertained by placing them on the platinum wire in the hot- 

 test part of the flame. The sample is of weak emissive power 

 when it is less luminous than the platinum wire, of a mean emis- 

 sive power when both appear about equally luminous, and of 

 strong emissive power when the intensity of the light which it 

 emits is greater than that from the platinum. Most solid bodies 

 emit a white light, others (as, for instance, erbia) coloured light. 



Some bodies, such as many osmium, carbon, and molybdenum 

 compounds, volatilize and separate out finely divided solid mat- 

 ter, which renders the flame luminous. Gases and vapours 

 always exhibit a smaller power of emission than fused substances, 

 and these generally less than solid bodies. The form of the 

 substance under examination must always be noted, as the emis- 

 sive power depends upon the nature of the surface : thus com- 

 pact alumina, obtained by slowly heating the hydrate, possesses 

 only a moderate emissive power, whereas the porous oxide pre- 

 pared by quick ignition of the sulphate possesses a high power 

 of emission. 



2. The melting-point is determined by using the six different 

 temperatures already mentioned. At every increase of tempera- 

 ture the bead is examined with the lens to see whether the vo- 

 lume is decreased or increased, whether bubbles are given off on 

 melting, whether, on cooling, the bead is transparent, and what 

 changes of colour it undergoes during the action of the heat or 

 on afterwards cooling. 



3. The volatility is ascertained by allowing equally heavy 

 beads of the substance, placed on a platinum wire, to evaporate 

 in the zone of fusion, and observing the time, by means of a me- 



