As long ago as 1924 the striking observation was made almost simultaneously by Hess and by Steenbock and Nelson that many foods could be made antirachitic by irradiation with ultraviolet rays. Some of the first successes in this line have already been referred to in earlier chapters: Hess's demonstration that his irradiated cottonseed and linseed oils were as potent as cod liver oil to cure rickets in rats; Steenbock's similar showing with his whole irradiated diet; and, particularly significant, Steenbock and Daniels' proof that even nonrachitic babies could have their calcium retention increased by feeding irradiated olive oil instead of ordinary olive oil. Daniels' later article is even more striking than this earlier work. She found her irradiated milk and her irradiated olive oil both more effective than cod liver oil. The reason for this superiority she is unable to explain. She suggests that the milk and the oil, which