The quote is part of a speech given by J.J. Thomson in 1916. The speech is partially reproduced (including your part) in the book "The life of Sir J.J. Thomson, O. M. : sometime master of Trinity College Cambridge" by Lord Rayleigh (1942). The original book is on the Internet Archive [1]. The speech is given at pages 198-200 and your quote appears approximately halfway in the quoted part of the speech [see 2].
Completely agree about JJ Thomson. Just happy to hear it was he who said it and not a that it was a quote by someone else falsely attributed to JJ Thomson because he is famous.
[1] Full biography: https://archive.org/details/b29932208
[2] Screenshot of relevant quote: https://imgur.com/a/qicgorD