Need help remembering CS Lewis Quote about Joy and Chemical Reactions

I know that there is a line by CS Lewis where he is talking about secular atheists have to try to forget their philosophy in order to find joy in life. The line goes something like... "But insofar as... you go from having a good time to having a great time... you must remember that love is merely a chemical reaction." There might also be a part about his wife, and how his love for her was nothing but a chemicial reaction if we presume that the secularist worldview is correct. If someone could give me the quote along with perhaps a reference to where it came from I would be very grateful.