10 POPULAE SCIENCE, 



disease is so strongly fixed in his mind, that he has expressed 

 himself ready to be the subject of experiment, by breathing a 

 diseased atmosphere, armed with no farther protection than a 

 mouth-and-nose filter of cotton-wool. 



Perhaps the utilitarian in a perfect sense will cavil at the 

 enunciation of spectrum analysis as a case in illustration of 

 the point I wish to illustrate namely, the practical applica- 

 tion, time being given, of the most seemingly useless disco- 

 veries. He may tell me, that although it may be very in- 

 teresting to know what the sun and stars are made of, and 

 though spectrum analysis would seem to have taught us this, 

 yet such knowledge is of no use to us. Looking at utility 

 from his point of view, we must perhaps give way to the prac- 

 tical man, and say, ' Agreed.' Kesponding to this considerate 

 deference, will the practical man kindly meet us half-way ? 

 Will he please to remember, that by aid of this spectral ana- 

 lysis Mr. Crooke discovered the new metal thallium ? Will 

 he admit that the discovery of any new metal belongs to his 

 utilities ? No, he says ; thallium is not a useful metal. Well, 

 I do not feel inclined to argue that point, the practical man 

 and I viewing the evidence under lights so totally diverse. 

 Not getting up a contest with him, I will proceed to describe 

 the facts and the general line of evidence on which this spec- 

 tral analysis is based. 



Everybody knows that when a sunbeam is passed through 

 a triangular prism, the beam is decomposed into three primary 

 colours : red, yellow, and blue. Newton described as many as 

 seven colours; but more modern experimenters have proved 

 all, save three, to be secondary mere mixed tints. So much 

 for colour; but the solar rays are not composed of light alone. 

 They have heat rays too, and another set of rays termed ' ac- 

 tinic,' to the operation of which latter chemical changes are 

 referable all the changes of daguerreotype and other sun- 

 painting, wrongly called < photography,' for example. It is 

 with the luminous part of the spectrum we have now to do. 



