370 CELESTIAL CHEMISTRY. 



Some 2,000 of these dark lines have been identified by 

 Professor Rowland as belonging- to iron vapour, for it must 

 be perfectly clear that they could not be due to a mere coin- 

 cidence : the mathematical probabilities against any such pure 

 coincidence are more than a million million millions to one. 

 We are therefore compelled to accept the conclusion that the 

 sun's atmosphere contains metallic iron. 



Similarly we are driven to conclude that the atmosphere 

 surrounding the sun contains numerous other elements whose 

 dark lines we see in the sun's spectrum, and we must also 

 conclude that the heat is sufficiently high to keep them all in 

 the state of vapour. Amongst these are the metals sodium, 

 calcium, barium, magnesium, zinc, chromium, cobalt, copper, 

 nickel, silver, manganese, aluminium, lead, bismuth, molyb- 

 denum, tin, and several others. All these, and more, coming, 

 as vapour, between the glowing nucleus of the sun and our- 

 selves, have, so to say, thrown their many lines of shadow 

 across the sunlight, and all these many shadows indicate the 

 presence of the substances that cast them. 



I may leave you to imagine — for words fail one to describe 

 it — how stupendous the heat on the sun's surface must be, in 

 order to be capable of converting these metals not merely into 

 the molten state, but actually into vapour. 



Nearlv half a century ago, the spectroscope, by its delicacy, 

 enabled Kirchhoff and Bunsen to discover two metals, pre- 

 viously unknown to science, in certain mineral waters; and a 

 short time after these discoveries Sir William Crookes 

 discovered a third new metal — thallium. All these metals, and 

 several subsequently discovered, demonstrated their existence 

 by their bright line spectra, but the most remarkable discovery 

 of those days was made by Sir Norman Lockyer about thirty 

 years ago. Just a few words in explanation. The brilliant disc 

 of the sun, as seen by us, is called the photosphere. Completely 

 surrounding this photosphere is a comparatively transparent 

 shell or envelope of coloured gas called the chromosphere : it 

 forms, we may perhaps say, the middle layer of what for 

 convenience may be spoken of as the sun's atmosphere. 

 Directing his spectroscope towards this chomosphere. Sir 

 Norman Lockyer noticed a line in the yellow region near that 

 of sodium — one which did not correspond with any metal or 

 element known on earth, and the conclusion was accordingly 

 inevitable that there existed, in enormous quantities, in the 

 chromosphere of the sun, an element whose existence had not 

 yet been traced anywhere else, and to this new element the name 

 helium was given, from the Greek word " helios," which means 

 "the sun." It was positively certain that helium was a new 

 element, and yet for over a quarter of a century, the 93 millions 

 of miles which separate us from the sun appeared to be the 

 closest contact that man could ever hope to have with helium. 

 In 1886 the yellow line of helium was found in the great nebula 

 in Orion, and only as recently as 1903 Sir William Ramsay, 

 Professor of Chemistry at University College, London, 

 obtained a gas from the mineral Cleveite, and this gas not only 

 proved to be a new element, but its spectrum exhibited exactly 



