142 



NA TURE 



February 13, 1896 



THE STORY OF HELIUM} 

 Chapter II. 



MY next chapter breaks new ground. I take you right 

 away from the celestial regions, we leave the sun 

 for a time, and I land you at Washington. I take you to 

 a room in the geological department there, and I intro- 

 duce you to one of the officials at work — a certain Dr. 

 Hillebrand. 



He is engaged upon the chemical examination of speci- 

 mens of the mineral uraninite from various localities. 

 The time was the year 1888. 



He deals with a little crystal such as I have in my 

 hand, he puts it in a vessel containing some sulphuric 

 acid and water. He finds that he gets bubbles, bubbles 

 of gas produced out of the crystal by means of the 

 sulphuric acid. Well, he collects this gas, gets a quantity 

 of It, tries to find out what it is, and he comes to 

 the conclusion that it is nitrogen, and he publishes 

 a statement that this gas obtained from the mineral 

 uraninite consists of nitrogen. 



This result was new. He thus writes about 

 it :— 



" In consequence of a certain observation " [the 

 one I have just referred to] "and its results, an 

 entirely new direction was given to the work, and 

 its scope wonderfully broadened. This was the 

 discovery of a hitherto unsuspected element in 

 uraninite, existing in a form of combination not 

 before observed in the mineral world." 



It is not needful for my story to follow Dr. 

 Hillebrand through all the painstaking and patient 

 labour he cut out for himself, to explain this ano- 

 malous behaviour. Needless to say he did not 

 omit to employ the spectroscope to test the nature 

 of the new gas. 



He writes^: — 



" In a Geissler tube under a pressure of 10 

 millimetres and less, the gas afforded the fluted 

 spectrum of pure nitrogen as brilliantly and as 

 completely as was done by a purchased nitrogen 

 tube. In order that no possibility of error might 

 exist, the tube was then reopened and repeatedly 

 filled with hydrogen, and evacuated till only the 

 hydrogen lines were visible. When now filled 

 with the gas and again evacuated, the nitrogen 

 spectrum appeared as brilliantly as before, with 

 the three bright hydrogen lines added." 



On this paragraph I may remark that it has long 

 been known that gases like nitrogen give us quite 

 distinct spectra at different temperatures — one 

 fluted, another containing lines. Which of these 

 we shall see in a tube will depend upon the pressure 

 of the gas and the electric current used. The fluted 

 spectrum of nitrogen is very bright and full of 

 beautiful detail in the yellow part of the spectrum ; 

 the line spectrum, on the other hand, is almost bare in 

 that region. 



Note well that it so happened that the pressure and 

 electric conditions employed by Dr. Hillebrand enabled 

 him to see the fluted spectrum. 



Dr. Hillebrand concludes his paper by pointing out 

 that — 



" The interest in the matter is not confined merely to a 

 solution of the composition of this one mineral ; it is 

 broader than that, and the question arises. May not 

 nitrogen be a constituent of other species in a form 

 hitherto unsuspected and unrecognisable by our ordinary 

 chemical manipulations ? And, if so, other problems are 

 suggested which it is not now in order to discuss." 



1 Continued from p. 322. 



2 " On the Occurrence of Nitrogen in Uraninite,' Bulletin, No. 78, U.S. 

 Geol. Survey, 1889-90, p. 55. 



Chapter III. 



Now, I have another part of the story to bring before 

 you. 



Following the recognised practice of story-tellers, I 

 have to change the scene ; instead of dealing with the 

 sun, I must take you still further afield and consider 

 some points connected with distant suns, which we call 

 stars, and the nebulaj which are so often associated with 

 those distant suns. A star appears to us as a point, but 

 a nebula appears to us as a surface, and the result is that 

 the method of investigating these two classes of heavenly 

 bodies, to get at the facts I have to bring before you, is 

 somewhat different. 



Let us deal, in the first instance, with the method of 

 photographing the spectrum of a star which appears to 

 us as a point. The best way of doing that is to use an 

 instrument represented in this diagram, which consists of 



x\0. 1372, VOL. 53] 



Fig. 8. — The objective prism. 



a telescope with an object-glass at one end in front of 

 which a prism of large dimensions is fixed, through which 

 the light has to pass before it enters the telescope at all. 



Hence on the photographic plate, instead of the point 

 formed by a star under ordinary conditions, owing to the 

 tearing asunder by the prism of the different coloured 

 rays of which the white light of the star is composed, 

 we get a fine line, along which is represented the com- 

 plete radiation, or complete system of hieroglyphics, from 

 red to violet. 



Another method is to employ a spectroscope of the 

 ordinary construction in connection with a large reflector. 

 The light from the star, which is grasped by the large 

 mirror, is thrown by a diagonal mirror inside the tube on 

 to the slit. In the case of the nebulae, of course allowing 

 first one part and then another to fall upon the slit of the 

 spectroscope, we are enabled to determine what light-notes 



