S6 



KNOWLEDGE. 



[April 1, 1800. 



information than much verbal description. Dr. Weinek 

 notes no new objects here. 



It may be added that this photograph slightly overlaps, 

 at its lower left-hand corner, that given of the district of 

 Maurolycus in Knowledgk for June, 1805. 



THE SPECTRUM OF HELIUM. 



By E. Walteii Maundkr, F.R.A.S. 



THE first vague glimpse of the spectrum which has 

 won so much attention during the last twelve 

 months would appear to have been caught by 

 Prof. Magriui at Milan, during the total solar 

 eclipse of 1812, July 8th. A momentary glance 

 at the corona through a tlint-glass prism showed him three 

 vivid colours, the colours which, following Sir David 

 Brewster's theory, were then considered primary — red, 

 yellow, and blue. No notice seems to have been talsen of 

 the observation ; the observer himself seems to have laid 

 no stress upon it. Spectrum analysis had not then made 

 sufficient advance for it to be understood ; yet it was none 

 the less the first faint forecast of fuller observations to 

 come. It was the evidence, could it have been then inter- 

 preted, that the " red flames," so often noticed round the 

 dark moon in a solar eclipse, belonged truly to the sun ; 

 that they were gaseous in character ; and that amongst the 

 glowing gases which made them up, hydrogen, and a gas 

 at that time stranger to us, were the most abundant. 



Twenty-six years after Prof. Magrini's observation, in 

 the celebrated eclipse of 18G8, August 18th, the same 

 three colours flashed out their light on the expectant 

 observers who, at Guntoor, Jamkandi, Masulipatam, and 

 Tenasserim, were watching the eclipse, armed not with 

 simple prisms merely, but with complete spectroscopes 

 attached to powerful equatorials. There is no need to quote 

 at length the accoimts which have been so often retold. 

 Suffice it — to take one observation amongst many — that 

 Captain Herschel, setting the slit of his spectroscope on a 

 brilliant prominence, needed but to give " a single glance, 

 and the problem was solved. Three vivid lines — red, orange, 

 blue ; no others, and no trace of a continuous spectrum." 

 The problem was solved— that is, so far as the question 

 was concerned that the prominences belonged to the sun, 

 and were gaseous in character. The nature of the gas was 

 a different matter, and here the evidence of the eclipse 

 itself was incomplete. M. Janssen was confident that 

 the red and blue lines were the lines of hydrogen ; other 

 observers thought it more or less probable ; most believed 

 the yellow line to belong to sodium : and so the matter 

 might have rested. But impressed with the extreme vivid- 

 ness of the prominence lines, Janssen resolved to look for 

 them after the eclipse was o\er, and was delighted to find 

 his anticipation realized, and that they could be seen in 

 full sunshine. Then it was an easy matter to establish 

 decisively the conclusion he had arrived at during the 

 eclipse, that several of the prominence lines were due to 

 hydrogen. But it became equally evident that the bright 

 yellow line was not coincident with either of the two dark 

 D lines due to sodium ; it was not, indeed, coincident with 

 loii/ dark line in the solar spectrum, and it lay further 

 towards the blue than D., by fully twice the distance that 

 separated the latter from its twm brother I)j. 



At first the new line, Dj as it was called, was supposed 

 by many to be a line of hydrogen produced only under 

 conditions which we are not able to reproduce in the 

 laboratory. It was as constant a feature of the chromo- 

 spheric spectrum as the lines of hydrogen ; it rose to the 

 same height from the sun. But more careful scrutiny 



showed tliat it did not by all means always respond to the 

 changes and displacements shown by C and F, the un- 

 doubted hydrogen lines ; and it was not long before it was 

 acknowledged to indicate the presence of a gas unknown to 

 us as yet hero, and on I'rankland's suggestion the name of 

 " helium " was assigned to it. 



Again a little over twenty-six years passed by without 

 our knowledge of " helium " receiving any addition, until 

 Prof. Ramsay's discovery of it, just a year ago, in the gas 

 obtained from the mineral cleveite. The story of that 

 discovery, and of the researches into the properties of the 

 gas which Prof. Uarasay and other physicists lost no time 

 in instituting, has already been well told in the pages of 

 Knowledue by Dr. McGowan (September, 1895). The 

 story of the complete study of its spectrum is of fully equal 

 interest and is of more recent date. The romance of 

 helium is not completed even yet. 



It will bo remembered that for a short time the identity 

 of the gas obtained from clcveite with true solar helium 

 was called in question. This was owing to the discovery 

 of Profs, liunge and Paschen, of Hanover," that the 

 bright yellow line from the new gas was not single (as the 

 D, line of the solar chromosphere was then supposed to be), 

 but double. Dr. McGowan has already described how the 

 observations of Dr. Iluggins and Prof. Hale speedily set 

 the question at rest by showing that the solar I);, was 

 double likewise, and so completely establishing Prof. 

 Ramsay's claim to have " run helium to earth." 



But this discovery of Runge and Paschen was but an 

 incident of the work which they were carrying on with the 

 most powerful of spectroscopic appliances, and with the 

 utmost thoroughness and care. The spectroscope was one 

 of Rowland's concave gratings, with a radius of curvature 

 of over twenty-one feet, a ruled surface of six inches, and 

 ruled with twenty thousand lines to the inch. The 

 photographs taken with this instrument rendered it 

 possible to measure the wave-lengths of the lines in the 

 great majority of cases to the thousandth of a tenth-metre, 

 the errors of the determinations (due mainly to the 

 unsuitability of some of the reference lines for measure- 

 ments of such delicacy) lying for the most part in the 

 third decimal place. ! 



In order to obtain the greatest possible brilliancy of 

 spectrum the vacuum-tube was used " end-on," an arrange 

 ment first employed to any great extent in this country by 

 Prof. Piazzi Smyth, then Astronomer Royal for Scotland. 

 By this arrangement, which necessitates a special form of 

 tube, the light from the entire length of the capillary 

 tube is concentrated on the slit instead of that from its 

 breadth only. The electrodes consisted of two clyinders of 

 aluminium foil, one in each of the two wide parts of the 

 tube, and pressing against two platinum wires, thus 

 allowing the capillary to be viewed end-on without inter- 

 ruption. The end of the tube nearest the slit of the 

 spectroscope was closed by a " window "—a flat piece of 

 glass, quartz, or fluor spar, according to the region of the 

 spectrum under study. 



A further essential for the work, and one far more 

 difficult to secure, was the purity of the helium gas. The 

 chief impurity was hydrogen gas. The helmm was 

 prepared in the same way as it was originally discovered 

 by Prof. Ramsay, i.e., by boiling cleveite with diluted 

 sulphuric acid. This was then mixed with a large quantity 

 of oxygen, and sparked continuously for several days uatil 

 a whole day's sparking caused no farther contraction. The 

 o.'cygen was next absorbed, and the remaining gas stored in 



* Siitnre of .June (ith, 189ij. 

 f Aslroplni-iienl Joiii-iinl, 189(i, Januarv. 



