BEFORE passing to an account of Jeans's technical and scientific achievements, it may be of interest to sketch the background of science in the days of his boyhood, more especially as Jeans himself was to write, at the age of fiftysix, a volume entitled The New Background of Science. And, as Jeans devoted the last eighteen years of his life to popular exposition, the best way of doing this would appear to be to take a brief survey of the state of popular science in the last half of the nineteenth century.

Let us take the lectures and writings of two renowned expositors of physics, Helmholtz in Germany and Tyndall in England.

The Popular Lectures on Scientific Subjects of Hermann von Helmholtz were published in an excellent English translation in two volumes in 1893. They consisted of addresses on sundry formal occasions, delivered to educated but not specialist audiences, and covered ground to which Helmholtz himself had made notable contributions. One group of addresses was concerned with the first law of thermodynamics (as it is now called), namely, the law of conservation of energy or, as Helmholtz termed it, the law of conservation of force. This great generalization appealed strongly to Helmholtz. It was the subject of his Carlsruhe address of 1862, ‘On the Conservation of Force’; he had emphasized it in his Konigsberg address of 1854 ‘On the Interaction of Natural Forces’, and he was to dwell on it again in his Innsbruck address of 1869, ‘On the Aim and Progress of Physical Science’. The possibility that the law of conservation of energy applied to all forms of energy had been outlined by Julius Robert Mayer in 1842; but it was the experiments of James Prescott Joule, published in 1843, which first established the strict equivalence of heat and mechanical energy. Joule's classical paper on this subject was dated 1849. It is evident from Helmholtz's insistence on the importance of this law that its power and generality were not fully realized by the audiences which he was addressing: It was necessary for him to pile example upon example.