COLOR AND CHEMICAL CONSTITUTION 119 



Our best modern instruments for work in the infra-red region 

 depend entirely on the heating effect. So sensitive indeed is the bolom- 

 eter, as devised by Langley, that a difference in temperature of one 

 five-hundred-thousandth of one degree F. can be determined and, by its 

 use in this region of the spectrum, rays of wave-length 100,000 A.U. 

 have been detected. The study of the ultra-violet region of the spec- 

 trum depends upon the sensitiveness of silver compounds and accord- 

 ingly on photographical measurements. As glass was found to absorb 

 the rays of shorter wave-lengths than 3,300 A.U., quartz lenses and 

 prisms must be used. Quartz absorbs rays of a wave-length less than 

 1,850 A.U., but fluorite may be used for rays down to a wave-length 

 of 1,000 A.U. Air itself exerts a powerful absorption for rays of 

 a wave-length of 2,000 A.U. and under, hence for these finer observa- 

 tions the apparatus must be exhausted and observations made in a 

 vacuum. In the photographical determinations of this region the 

 greatest care must be taken in preparing the silver bromide plates. 

 No gelatine can be used upon the plates as it is found to exert a strong 

 absorption for the shortest rays. The sensitive plates are usually 

 made by precipitating silver bromide in a solution over a glass plate 

 and allowing the precipitate to settle slowly upon this plate. When 

 these sensitive films are colored, the plate becomes more sensitive to 

 the rays which the dye absorbs. 



The principle established by Kirchhoff was applied with intense 

 vigor to the study of all the lines of the solar spectrum. The intro- 

 duction of various substances into a flame was found to give spectra 

 of many colored lines, but these were always definite for each and 

 every substance examined. The lines in these discontinuous spectra 

 were seen in many cases to have their exact counterpart in certain of 

 the Fraunhofer lines and consequently the existence of the particular 

 element producing them may be assumed in the solar atmosphere. In 

 this manner, the chemical composition of the sun's atmosphere has 

 been determined, and even new elementary substances discovered there- 

 in by the selection of certain lines or groups of lines, unaccounted for 

 by any element previously studied. The line-spectra of many elements 

 are readily obtainable at low temperatures, but for iron and similar 

 metals a far higher temperature is required, as, for example, that of 

 the arc. For gases a strongly induced electric current (one of high 

 tension) is necessary. In the arc spectrum of iron over 2,000 lines 

 are observed, whereas the spectrum obtained at the flame temperature 

 consists of only a few bands and lines. At the hottest portion of the 

 spark the iron spectrum shows the same lines as in its arc spectrum, 



