23 



The power of heat alone to render a gas which is almost colour- 

 less as red as blood without decomposing it, is in itself a most singu- 

 lar result ; and the author succeeded in rendering the same pale 

 nitrous acid gas so absolutely black by beat, that not a ray of the 

 brightest summer's sun was capable of penetrating it. In making 

 the experiment, the tubes frequently exploded ; but by using a 

 mask of mica and thick gloves, and placing the tubes in cylinders of 

 tinned iron with narrow slits to admit the light, there is little danger 

 of any serious accident. 



The author then points out various practical applications which 

 may be made of this discovery, especially its substitution for the 

 more difficult process, when Fraunhofer's lines in the solar spectrum 

 are employed, of determining the dispersive powers of substances. 

 Since the absorptive action, by increasing the thickness of the 

 medium, generally enlarges the lines already defined, these may be 

 rendered as distinct as may be required, which it is impracticable to 

 do with the solar lines, and hence the difficulty of applying these to 

 useful purposes. The lines in the sun's light, and those of the 

 nitrous acid gas spectrum, when directly compared, have a strong 

 analogy ; but in order to establish it completely, the author found 

 Fraunhofer's map of the solar spectrum insufficient, and was induced 

 to undertake the laborious task of going over the whole ground. 

 By dint of perseverance, and by the use of some original methods, 

 he has been enabled, with very inferior instruments, to distinguish 

 about 2000 lines, instead of the 354 which Fraunhofer had laid 

 down. 



The author watched narrowly the state of the defective solar lines 

 at different seasons of the year, in order to observe if any change 

 took place in the combustion by which the sun's light is generated, 

 or in the solar atmosphere through which it must pass. Such 

 changes he found to be very general in every species of terrestrial 

 flame. The definite yellow rays which exist in almost all white 

 lights, flicker with a variable lustre, and analogous rays in the green 

 and blue spaces proceeding from the bottom of the flame, exhibit the 

 same inconstancy of illumination. In the course of the winter obser- 

 vations, he observed distinct lines and bands in the red and green 

 spaces, which at other times wholly disappeared ; but a diligent com- 

 parison of these observations soon showed that these lines and bands 

 depended on the proximity of the sun to the horizon, and were pro- 

 duced by the absorptive action of the earth's atmosphere. 



The atmospheric lines, as they may be called, or those lines and 

 bands which are absorbed by the elements of an atmosphere, have 



