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Capt. Abney and Col. Festing. [Jan. 15 



should rather be said an appreciation — of this fact would be of practical 

 use. In such a case the visible radiation coming through a combina- 

 tion of cobalt and orange glass might be successfully employed. 



Curve IV is the absorption-spectrum thermogram of a green glass, 

 which obtains its colour from a combination of cupric oxide and 

 iron oxides. An examination of the prismatic spectrum photo- 

 graphed through such glass has led us to suspect that green glass is 

 transparent to certain rays of very low refrangibility. The thermo- 

 gram fully confirms this. Some four years ago one of us proposed to 

 the Meteorological Committee of the Royal Society the use of green 

 glass for thermometer bulbs, in the belief that it totally absorbed the 

 infra-red rays. This is now found to be a mistake, and some other 

 colouring matter must be sought for for this purpose. 



Fig. II illustrates some interesting facts in connexion with absorp- 

 tion. Curves I and VI, as already stated, are the thermograms of the 

 lamp without and with an empty glass cell in front of the slit. 

 Curve VII is the absorption thermogram of benzene. As far as 

 Scale No. 6 there is no appreciable absorption, but from that point 

 there is a drop in the curve, which proves to be coincident with the 

 radical band of the group of benzene compounds, which we mapped 

 photographically (see "Phil. Trans.," 1882). At a point between 

 7 and 8 on the scale absorption again commences, and although of 

 some small extent, is appreciable as far as the end of the thermogram, 

 thermal action being traceable nearly down to Scale No. 20, or wave- 

 length 27,000. This point we presume to consider of some importance, 

 for we have shown that there is reason to believe the existence of some 

 compound of the aromatic series between us and the sun, as the 

 radiant band is coincident with a very strong line in the solar spec- 

 trum. The solar spectrum as determined by the thermopile being 

 coterminous in this direction with that of the incandescence lamp, had 

 it been found that benzene cut off the lower part of the latter, our 

 belief as to the indications of benzene in the solar spectrum would 

 have been much shaken. 



Curve VIII is the thermogram of a deep-coloured solution of iodine 

 in alcohol. This terminates near Scale No. 16. We found a slight 

 dip near Scale No. 6 ; this was too slight to be conveniently shown on 

 the diagram. It is coincident in position with the alcohol radical 

 band. The thermogram of uncoloured alcohol is the same as that of 

 Curve VIII, with the exception that some further thermal action in 

 the visible part of the spectrum has to be registered. 



Curve IX is the thermogram of the absorption-spectrum of a satu- 

 rated solution of common salt in water. Rock salt, as is well known, 

 allows nearly all radiation to pass, and it seemed desirable to ascer- 

 tain what effect it would have in solution. The effect of the salt 

 appears to have been a mitigation of the absorptive power of the 



