522 Sir DAVID BREWSTEB on the Lines of the Solar Spectrum, 



traversing the spectrum. As no such lines appeared in the spec- 

 tra of white flames, FRAUNHOFER considered them as having 

 their origin in the nature of the light of the sun. The strongest 

 of these lines were seen in the spectra of the Moon, Mars, and 

 Venus, and, by means of very fine instruments, he was able to 

 detect one or two of them with other new h'nes in the spectra 

 of Sirius and Castor. 



Such was the state of the subject, when I made the experi- 

 ment already referred to on nitrous acid gas. Upon examining 

 with a fine prism of rock-salt, with the largest possible refracting 

 angle (nearly 78), the light of a lamp transmitted through a small 

 thickness of the gas, whose colour was a very pale straw yellow, 

 I was surprised to observe the spectrum crossed with hundreds 

 of lines or bands, far more distinct than those of the solar spec- 

 trum. The lines were sharpest and darkest in the violet and 

 blue spaces, fainter in the green, and extremely faint in the yel- 

 low and red spaces. Upon increasing, however, the thickness of 

 the gas, the lines grew more and more distinct in the yellow and 

 red spaces, and became broader in the blue and violet, a general 

 absorption advancing from the violet extremity, while a specific 

 absorption was advancing on each side of the fixed lines in the 

 spectrum. It was not easy to obtain a sufficient thickness of gas 

 to develope the lines at the red extremity, but I found that heat 

 produced the same absorptive power as increase of thickness, and, 

 by bringing a tube containing a thickness of half an inch of gas 

 to a high temperature, I was able to render every line and band 

 in the red rays distinctly visible. 



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

 colourless, as red as blood without decomposing it, is in itself a 

 most singular result ; and my surprise was greatly increased when 

 I afterwards succeeded in rendering the same pale nitrous acid 

 gas so absolutely black by heat, that not a ray of the brightest 

 summer's sun was capable of penetrating it. In making this ex- 

 periment, the tubes frequently exploded, but, by using a mask 



