Prof. Tyndall on the Physical Basis of Solar Chemistry. 151 



chlorides of strontium, calcium, lithium*, and other metals; each 

 salt gave the bands due to the metal. Different salts were then 

 mixed together and rammed into the holes in the carbon ; a spectrum 

 was obtained which contained the bands of them all. 



The position of these bright bands never varies ; and each metal 

 has its own system. Hence the competent observer can infer from 

 the bands of the spectrum the metals which produce it. It is a lan- 

 guage addressed to the eye instead of the ear; and the certainty 

 would not be augmented if each metal possessed the power of 

 audibly calling out, " I am here !" Nor is this language affected 

 by distance. If we find that the sun or the stars give us the bands 

 of our terrestrial metals, it is a declaration on the part of these orbs 

 that such metals enter into their composition. Does the sun give 

 us any such intimation ? Does the solar spectrum exhibit bright 

 lines which we might compare with those produced by our terrestrial 

 metals, and prove either their identity or difference? No. The 

 solar spectrum, when closely examined, gives us a multitude of fine 

 dark lines instead of bright ones. They were first noticed by Dr. 

 Wollaston, were investigated with profound skill by Fraunhofer, and 

 named from him Fraunhofer's lines. They have been long a standing 

 puzzle to philosophers. The bright lines which the metals give us 

 have been also known to us for years ; but the connexion between 

 both classes of phenomena was wholly unknown, until Kirchhoff, 

 with admirable acuteness, revealed the secret, and placed it at the 

 same time in our power to chemically analyse the sun. 



We have now some hard work before us ; hitherto we have been 

 delighted by objects which addressed themselves rather to our aesthetic 

 taste than to our scientific faculty. We have ridden pleasantly to 

 the base of the final cone of Etna, and must now dismount and march 

 wearily through ashes and lava, if we would enjoy the prospect from 

 the summit. Our problem is to connect the dark lines of Fraunhofer 

 with the bright ones of the metals. The white beam of the lamp is 

 refracted in passing through our two prisms, but its different com- 

 ponents are refracted in different degrees, and thus its colours are 

 drawn apart. Now the colour depends solely upon the rate of oscil- 

 lation of the particles of the luminous body, — red light being pro- 

 duced by one rate, blue light by a much quicker rate, and the colours 

 between red and blue by the intermediate rates. The solid incan- 

 descent coal-points give us a continuous spectrum ; or, in other 

 words, they emit rays of all possible periods between the two ex- 

 tremes of the spectrurr. They have particles oscillating so ; s to 

 produce red ; others to produce orange ; others to produce yellow, 

 green, blue, indigo, and violet respectively. Colour, as many of 

 you know, is to light what pitch is to sound. When a violin-player 



* The vividness of the colours of the lithium spectrum is extraordinary : 

 it contained a blue band of indescribable splendour. It was thought by 

 many, during the discourse, that I had mistaken strontium for lithium, as 

 this blue band bad never before been seen. I have obtained it many times 

 since; and my friend Dr. Miller, having kindly analysed the substance 

 made use of, pronounces it chloride of lithium. — J. T. 



