This reminiscence of JS Bach is from a letter by CPE Bach to JN Forkel, probably from December 1774.
The account of my late father’s life given in Mizler’s journal is, thanks to my participation, the most complete. The list of his keyboard compositions given there omits the following: fifteen two-part Inventions and fifteen three-part Sinfonias, as well as six short preludes. Regarding the sacred music of the deceased, it may be said that he worked on it in all humility according to the sense of the text, neither comically distorting the words, nor highlighting individual words at the expense of the overall meaning, a common fault, often suggesting laughable ideas which may elicit admiration both from the ignorant and from those who wish to appear knowledgeable. There has never been anyone who could inspect organs so acutely and yet with such honesty. His knowledge of organ-building was encyclopaedic….When trying out an organ, the first thing he did was to remark jovially, “Before I do anything else, I need to know whether the organ has got good lungs”, and in order to find this out, he would pull out all the stops and play in as many parts as possible. At this point the organ-builders would often turn quite pale with fright. Thanks to his great skill in harmony, he was able, on more than one occasion when accompanying a trio and feeling in good humour, to improvise a fourth part over the sparsely figured bass part in front of him, as long as he knew that the composer of the trios would not take amiss….
CPE Bach, letter to JN Forkel, probably Dec. 1774, quoted in:
The Bach Reader. A Life of Johann Sebastian Bach in Letters and Documents, ed. HT David and A Mendel, New York, 1945, rev. 2nd edn., 1966, pp. 275ff.
Note
Lorenz Christoph Mizler (1711-78 ) was a German writer on music.
Theory of Music 