I blogged a while back about Thomson’s 1913 Rays of Positive Electricity, which is available on-line in various formats. I recently found that the 1921 Rays of Positive Electricity Second Edition is also available. Thomson notes in the preface that
The hope expressed in the first edition that the method of Positive Rays would be of service in connection with important chemical problems has been fulfilled to a remarkable extent by the researches of Mr. Aston and others on the determination of atomic weights and the detection of isotopes. I am convinced that as yet we are only at the beginning of a harvest of results which will elucidate the process of chemical combination, and thus bridge over the most serious gap which at present exists between Physics and Chemistry.
I am trying out a Wiki version of the 1913 book. It looks good, but the markup is time consuming.