SKETCH OF LAVOISIER. 549 



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, send- 

 ing 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 ad- 

 mitted 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, con- 

 cerning 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 solu- 

 bility 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, facil- 

 ity 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 him- 

 self 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 imme- 

 diately 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 atmos- 

 phere 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 



