In 1839 Faraday gave five Friday discourses and a course of eight afternoon lectures on the non-metallic elements. In 1840 he gave three Friday discourses and seven lectures on chemical affinity. But in the summer came the serious breakdown alluded to on page 75. He did no experimental work after September 14th, nor indeed for nearly two years. Even then it was only a temporary return to research to investigate the source of the electrification produced by steam in the remarkable experiments of Mr. (afterwards Lord) Armstrong. He proved it to be due to friction. This done, he continued to rest from research until the middle of 1844, though he lectured a little for the Royal Institution. In 1841 he gave the juvenile lectures. In 1842 he gave two Friday discourses, one of them being on the lateral discharge in lightning-rods. He also gave the Christmas lectures on electricity. [Sidenote: END OF SECOND ACTIVE PERIOD.] In 1843 he gave three Friday discourses, one of which was on the electricity generated by a jet of steam; and repeated the eight afternoon lectures he had given in 1838. In 1844 he gave eight lectures on heat and two Friday discourses. He also resumed research on the condensation of gases, and vainly tried to liquefy oxygen and hydrogen, though he succeeded with ammonia and nitrous oxide. During these years of rest he also did a little work for Trinity House, chiefly concerning lighthouses and their ventilation. CHAPTER V. SCIENTIFIC RESEARCHES: THIRD PERIOD. Throughout the fruitful ten years of Faraday’s middle period two magistral ideas had slowly grown up in his mind, and as he let his thought play about the objects of his daily activities, these ideas possessed and dominated him as no newly suggested idea could have done. They were the correlation and inter-convertibility of the forces of nature, and the optical relations of magnetism and electricity. During the period of enforced rest, from 1839 to 1844, these ideas had been ever with him. His was a mind which during times of quiet brooding did not cease to advance. In silence his thoughts arranged themselves in readiness for the next period of activity, and his work, when it began again, was all the more fruitful for the antecedent period of cogitation. [Sidenote: OPTICAL ANALYSIS.]