Thomson was never without his note-book. Even in his journeys to London, when he usually took the night train to save time, his mind was active, and the green-book was in frequent requisition to receive thoughts that occurred relative to problems that engaged his attention. Unlike many mortals, he was able to sleep soundly on those night trips, although in the early days he had none of the luxuries of traveling which we consider indispensable to our comfort. Helmholtz records that, being on the _Lalla Rookh_ on one occasion, Thomson "carried the freedom of intercourse so far that he always had a mathematical note-book with him; and as soon as an idea occurred to him, he began to reckon right in the midst of company." This reminds us of the answer which Newton gave to a friend who asked him how he accomplished so much. "By constantly thinking of it," was the brief reply. Concentration of the faculties is necessary for all good work; a distracted mind never achieved anything of value in philosophy, in science, in religious worship. Concentration is like a convex lens, which brings rays to a focus; whereas distraction is like a concave lens, which breaks them up into a number of divergent and scattered elements. On leaving Cambridge in 1845, Thomson proceeded to London, and was warmly received by Faraday, then of world-wide reputation. He next went to Paris, where, in the laboratory of Regnault, he devoted himself to original research, under the direction of that great and accurate physicist who was then carrying out his classic work on the thermal constants of bodies. The year 1846 marks an epoch in Thomson's life; for, in that year, he was chosen to succeed Nichol, his friend and master, in the chair of natural philosophy in the University of Glasgow. Though only in his twenty-second year, he chose for the subject of his inaugural address the age of the earth, a subject which continued to have a life-long interest for him because of its very fascination, and perhaps, too, because of the opposition which his views aroused on the part of biologists and geologists. These demanded untold æons for the original fire-mist to cool down and form a spinning globe fit to be the abode of organic life, whereas Thomson endeavored to show the weakness of the arguments which they advanced to uphold their claim for unlimited time. Basing his estimate on the rate of increase of temperature as we go below the earth's surface, he concluded that the earth required from 100 to 200 million years, and probably less, to cool from its molten state to its present condition.