The Discovery of Argon

Argon Apparatus
A picture of the Argon Apparatus taken from his notebooks. You may want to compare this with that in his book 'The Gases of the Atmosphere' on the next page.  

On April 19th 1894 Ramsay heard Lord Rayleigh lecture to the Royal Society, when he pointed out that nitrogen isolated from the air had a density slightly higher than that of nitrogen prepared from chemical sources. A litre of pure nitrogen gas generated from a chemical reaction weighed 1.2505 g. On the other hand, a litre of nitrogen gas generated from air by removing oxygen, carbon dioxide, and water vapour weighed 1.2572 g (at the same temperature and pressure). Rayleigh thought that this might be due to the presence of a light impurity in the former (Rayleigh's paper is available online ).
But Ramsay thought that it might be due to the presence of a heavy impurity in the 'atmospheric' nitrogen. He was an enthusiast for the Newland/Mendeleev Periodic Table and we have Mendeleev's signature in one of our books of visitors. He thought that there might be an unrecognised new element hiding in the air, and that there might be room for this in a new Group at the end of the Periodic Table. When examinations had finished, Ramsay set about attempting to isolate this 'impurity'.

Periodic Table
The Periodic Table before Ramsay's discovery.