Blue light. I think some time ago, in the 1800's, blue light made everything grow better, and cured everything. Makers of blue glass did very well, and you can still see some old blue glazed sun porches sometimes. There's a cool book called Banvard's Folley by Paul Collins that examines some forgotten big errors of science, and some other things that just didn't work out. Extremely fun and well written book if you're an utter dork like me.