Scientific Lectures. 167 



their constitution must be similar to that of the sun ; that is, thej" 

 must be incandescent balls surrounded by an atmosphere ; and 

 although we have not as yet been able to form in regard to their 

 constitution the same definite conclusions we have reached with 

 regard to our sun, yet we think we have obtained sufficient evidence 

 to say tliat as one star differs from another star in glory, they also 

 differ in chemical composition. I have only been able to sketch in 

 this lecture a rude outline of the course of reasoning by which we 

 have arrived at some knowledge of the chemical composition of the 

 sun, and by which we yet hope to analyze the most distant stars. I 

 have endeavored to present the subject as simply and directly as I 

 could, thinking that the argument would be most impressive in its 

 greatest simplicity. Since the discovery of the law of gravitation, 

 there has been no scientific discovery which promises so greatly to 

 extend the boundaries of our knowledge as this. Newton gave ua 

 the rule by which we can measure the magnitudes and distances 

 of the heavenly bodies ; and now Kirchhoff and his collaborators 

 have found the test by which we may analyze the material of which 

 they are made. Until this great discovery, chemistry has been a 

 terrestial science, and the glory of the heavens has been the study of 

 astronomy alone. But now these sciences, seemingly separated bj 

 the widest gulf, have been most closely allied, and we have begun to 

 study the chemistry of the universe. [Applause.] But we have only 

 begun. The little which has been as yet revealed plainly shows us 

 how much there is still to learn, and we hold our knowledge with 

 great caution and humility. I shall not ask you to accept these 

 remarkable conclusions wholly on trust ; but having developed the 

 theory of the subject, as well as my narrow limits would permit, I 

 now propose to bring it to the test of experiment. 



Prof. Cooke proceeded to throw upon the screen, by means of the 

 electric light and a prism, first, the continuous spectrum of pure white 

 light ; then the bands produced by the vapors of copper, of lead, and 

 of salt; the latter showing the sodium line beautifully clear and 

 bright. Next he showed the bands produced by lithium, then of 

 thallium, one of the new metals discovered by means of the spectro- 

 scope ; then of brass, showing the zinc and copper lines combined. 

 He then showed again the continuous spectrum, from white light, 

 and interposing the vapor of salt, the sodium line appeared beautifully 

 distinct as a dark line. He then projected upon the screen the image 

 of the points producing the electric light ; the lower point which 



