74 PROGRESS OF SCIENCE IN THE CENTURY. 



discover a heavier gas in the atmosphere. In tho 

 meantime, and independently, Prof. W. Ramsay dis- 

 covered the same gas by removing the nitrogen by 

 means of red-hot magnesium. Combining their re- 

 sults, the two investigators published their memoir 

 on Argon, " which will go down to posterity among 

 the greatest achievements of an age renowned for its 

 scientific activity " (Meldola). 



Argon is an extraordinarily inactive or chemically 

 indifferent gas of great density; occurring along with 

 atmospheric nitrogen, forming about 8 or 9 per cent, 

 of the volume. It can be separated by incandescent 

 magnesium or by the continued action of the electric 

 spark, and in the latter way Cavendish seems actually 

 to have produced it a hundred years ago ! Alone or 

 along with helium it has been found in natural 

 waters, in minerals, and in a meteorite. It is not 

 known to form combinations, and it does not fit in 

 well with the periodic system, so that its real nature 

 remains the subject of enquiry. That it is truly 

 an element is suggested by the distinctness of its 

 electric spark spectrum and by the discovery that 

 the molecule is monatomic, but the possibility re- 

 mains that it is a mixture of monatomic gases. 



Helium. The facts in regard to the discovery of 

 helium are not less interesting. In 1868 Frankland 

 and Lockyer had observed a particular line D in the 

 solar spectrum which they attributed to the presence 

 of an element helium then unknown upon the 

 earth. It was also recognised in the spectrum of 

 Orion and other fixed stars. Subsequently the line of 

 helium was seen by Palmieri (1882) in the lava of 

 Vesuvius, and Hildebrand observed in 1891 what 

 were probably its lines in a spectrum of the nitrogen 

 gas which he got by heating or otherwise treating 



