An intimate friend of Faraday once described to me how, when Faraday was endeavouring to explain to Gladstone and several others an important new discovery in science Gladstone's only commentary was “but, after all, what use is it?” “Why, sir,” replied Faraday, “there is every probability that you will soon be able to tax it!”

By William Edward Hartpole Lecky, in Democracy and Liberty (1899, p. xxxi). Lecky's account is then subsequently quoted by R. A. Gregory in Discovery Or The Spirit And Service Of Science (1918, p. 3). Note that Lecky's account makes no reference to electricity. Subsequently, Alan Mackay (in The Harvest of a Quiet Eye : A Selection of Scientific Quotations, 1977, p. 56) gives the variant, "One day, sir, you may tax it", and adds the embellishment that the discovery is electricity. Mackay claims his source is Gregory (Discovery), but Gregory's quote differs and and makes no mention of electricity. Oxford Essential Quotations correctly refers to Lecky but also incorrectly adds the embellishment that the discovery was electricity. According to Snopes in "Long Ago and Faraday", it is most likely an invented quotation, but that Snopes article doesn't seem to be aware of Lecky's account.