the circle of his associates to his teachers and fellow-students; and, pleading that his health required it, he put himself upon an exclusive milk diet. Some of his friends seem to have believed that his health was really giving way; and M. de Troncq, sending him a dish of gruel, advised him in 1763 to be temperate in his studies, and to believe that "a year longer on the earth is worth more than a hundred in the memory of men."
Among his particular friends was Guettard, who had been admitted to the Academy as a botanist in 1743, but had afterward devoted himself to geology and mineralogy. He had already traveled in France and other countries in the interest of a plan he had conceived for making geological maps, upon which the kind of soil, mines, and quarries should be indicated by special marks. In connection with Guettard, Lavoisier made extensive excursions during three years through different parts of France. At the same time he studied the gypsum of the environs of Paris, concerning which he presented, in 1765, the first of the valuable series of memoirs with which he was to enrich the journals of the Academy of Sciences during nearly the next thirty years. His investigation included the varieties of the mineral and their solubility in water, and the cause of the setting of plaster, which he was the first to explain.
The Academy having, in 1765, offered a prize of two thousand livres for an essay on "the best means of lighting at night the streets of a large city, combining clearness of illumination, facility of service, and economy," Lavoisier resolved to compete for it, and began at once a series of experimental studies on the subject. In order to make his vision more sensitive to slight differences in the intensity of light, he hung his room in black, darkened it, and confined himself within it for six weeks, without permitting himself to look upon daylight for an instant. The two thousand livres were divided by the Academy among three competitors, who had incurred considerable expense in their experiments, while it gave a special distinction to Lavoisier's memoir by awarding the king's gold medal to the author, for which a public session was given.
The geological excursions with Guettard were resumed immediately after the conclusion of this transaction. The intervals of leisure were given to reading, studying, and making notes; among the fruits of which was an inquiry into the matter of fire and the nature of its elements. At first Lavoisier fancied that air was only water reduced to vapor, or rather water combined with the matter of fire; but this gave way at once to the conception of an atmosphere having an existence of its own and containing the fiery fluid and water in solution. Guettard's plan for a mineralogical atlas of France having been adopted by Minister Bertin, Lavoisier was