LECTURES. 45 



pounds the best means of answering the question are removed. 

 Another method, depending on Dulong and Petit's Law gives 

 untrustworthy results for gases. So that resource was had to the 

 indirect determination of the ratio of its specific heats at constant 

 volume and constant pressure. If a gas has two atoms in its 

 molecule this ratio should be i"4i, if only one atom 1.6. The 

 latter number was found and hence the atomic weight of Argon is 

 considered to be 40. A great difficulty now arises. There is no 

 room in this list of elements in Mendeleeffs table for an element of 

 this atomic weight, as the place is already occupied by a well 

 known element. The method of obtaining Specific Heats or the 

 Atomic Theory itself must therefore be seriously questioned. 



Thus the discovery of Argon and the investigation of its 

 properties has profoundly disturbed the very foundations of our 

 present Chemical Theory. 



" HELIUM." 



The history of this substance dates from 1868. In that year 

 Professor Norman Lockyer in subjecting the blood-red light visible 

 when the Sun was just getting eclipsed to spectrum analysis 

 thought he observed a yellow line very near but not coincident 

 with either of the yellow lines characteristic of the spectrum of 

 Sodium. 



On his return from India, where the observations were made, 

 he further investigated this line but was unable to find any known 

 substance, which would produce it artificially. 



Astronomical inventions soon enabled observers to see this 

 light without waiting for an eclipse. The result was to enable 

 Professor Norman Lockyer to assert that they were due to some 

 substance unknown on Earth but existing as a gas in the Sun, in 

 nebulae and in very hot stars. This gas he called Helium and he 

 inferred that it was a very light gas. 



In 1888, Hillebrand, during a research on the rare mineral 

 Uraninite found that an inert gas was evolved on the addition of 

 Sulphuric Acid. It obeyed the tests for Nitrogen and under the 

 conditions be employed gave, as he thought, the spectrum of 

 Nitrogen. 



In March, 1895, the attention of Professor Ramsay was drawn 

 to this peculiar fact, and as Nitrogen is never liberated from its 

 compounds by Sulphuric Acid, he at once suspected that it had 



