RECENT ADVANCES IN SCIENCE 363 



the two cases, namely chemical dissociation of an iron com- 

 pound by the action of heat, a mode to which he gives the name 

 thermo-chemical excitation. The character of the spectrum is 

 independent of the nature of the iron compound, which is 

 acted on by the thermal forces in either flame or furnace, 

 chlorides, oxides, etc., all giving the same kind of spectrum in 

 either of these sources at a given temperature. Thermo- 

 chemical excitation is quite distinct from chemical excitation, 

 which takes place at a comparatively low temperature in the 

 explosion region of the air-coal-gas flame, since the spectra 

 emitted by the compounds of iron in this region differ completely 

 from those mentioned above. The author makes some inter- 

 esting observations on the condition of the iron atom during 

 such thermo-chemical excitation, the gist of which is a belief 

 on his part that the iron atom during such thermo-chemical 

 excitation is not freed from the other atoms of the compound 

 molecule even up to a temperature of 2,700° C, although no doubt 

 the relative positions of these atoms are much disturbed. The 

 spectra are so restricted in range, although very intense, as to 

 suggest that the atoms emitting them are not so free as the 

 atoms which, under the action of the powerful electric forces 

 prevailing in the arc and condenser spark, emit the well-known 

 and wide-ranging iron spectrum. A further paper on the 

 " Origin of the line spectrum emitted by iron vapour in an 

 electric tube furnace at temperatures above 2,500° C," by the 

 same author, is printed in the October number of the Phil. Mag., 

 and adds further support to the hypothesis, leading to a dis- 

 cussion of five possible modes of excitation of the atoms of iron ; 

 one thermal, one chemical, two thermo-chemical, and one 

 electrical. 



The July number of the Proc. Roy. Soc. contains two short 

 papers by Prof. Fowler and C. C. L. Gregory, on the identifica- 

 tion of the ultra-violet band of Ammonia and group P in the 

 ultra-violet of the solar spectrum giving proof of the presence of 

 ammonia in the sun, and on the presence in the solar spectrum 

 of the water-vapour band ^3064, as furnishing further evidence 

 of the existence of oxygen in the sun. 



In the October number of the Proc. Roy. Soc. Prof. Collie 

 and Dr. Watson give an account of some experiments on the 

 appearance of lines due to the cathode metal in an electric 

 discharge through an inactive gas. The metal chosen for the 



