Here's something written by JJ Thomson on the "rays of positve electricity" which got me thinking about isotopes:
Proceedings of the Royal Society A 89, 1-20 (1913) [as excerpted in Henry A. Boorse & Lloyd Motz, The World of the Atom, Vol. 1 (New York: Basic Books, 1966)]
With regard to the sensitiveness of the positive ray method, I have made, as yet, no attempt to design tubes which would give the maximum sensitiveness, but with the tubes actually in use there is no difficulty in detecting the helium contained in a cubic centimetre of air, even though it is mixed with other gases, and I have not the slightest doubt a very much greater degree of sensitiveness could be obtained without much difficulty.
I will illustrate the use of the method by some applications. The first of these is to the detection of rare gases in the atmosphere. Sir James Dewar kindly supplied me with some gases obtained from he residues of liquid air; the first sample had been treated so as to contain the heavier constituents. The positive-ray photograph gave the lines of xenon, krypton, argon, and a faint line due to neon; there were no lines on the photograph unaccounted for, and so we may conclude that there are no heavy unknown gases in the atmosphere occurring in quantities comparable with that of xenon.
The second sample from Sir James Dewar contained the lighter gases; the photograph shows that, in addition to helium and neon, there is another gas with an atomic weight about 22. This gas has been found in every specimen of neon which has been examined, including a very carefully purified sample prepared by Mr. E. W. Watson and a specimen very kindly supplied by M. Claud, of Paris. ... The substance giving the line 22 also occurs with a double charge, giving a line for which m/e = 11. There can, therefore, I think, be little doubt that what has been called neon is not a simple gas but a mixture of two gases, one of which has an atomic weight about 20 and the other about 22. The parabola due to the heavier gas is always much fainter than that due to the lighter, so that probably the heavier gas forms only a small percentage of the mixture.
Thomson was also joined by Aston. They went on to develop Soddy's ideas on isotopes.
The two forms of neon were called isotopes by Frederick Soddy. One of Thomson's students, Frederick Aston, developed Thomson's idea of multiple species of an element, and in 1919 Aston produced the first mass spectrograph (an instrument that determined isotopic ratios), ancestor of today's mass spectrometer.
In 1908 Aston's father died, leaving Aston enough money to travel around the world. The following year Professor J.J. Thomson invited Aston to move to the Cavendish Laboratory and work as Thomson's assistant. Aston had been recommended to Thomson by his former teacher Poynting, and was happy to accept the Cambridge post which would leave him more time for research. Since his discovery of the electron Thomson had been analysing positive rays, and had developed a method of measuring atomic weights by using combinations of magnetics and electric fields to produce curves on a photographic plate. Aston helped to further refine these experiments.
Thomson and Aston found an unexpected effect when they investigated the element neon. Instead of showing only one curve on the photograph, it showed two, suggesting that the element was made up of two different types of atoms which were chemically identical but of different mass, named isotopes. This interested Aston, and he devoted the rest of his career to developing the mass spectrograph capable of accurately measuring these chemical isotopes.
This whole thing has got me thinking about oxygen. Under standard conditions, one volume of oxygen gas is 16 times heavier than the same volume of hydrogen gas. According to the Periodic Table this gives oxygen an atomic weight of 16. When decomposed to make water though, it is one volume of hydrogen plus a half-volume of oxygen. I think that this suggests that oxygen with an atomic weight of 16, is actually made up of two components each with an atomic weight of 8. Oxygen has the atomic number 8, and I wonder if oxygen's true atomic weight is also 8, and not 16. Is it possible that the oxygen gas we see with an atomic weight of 16 is actually made up of two equal isotopes?
Many thanks also: