It was partly due to Laurent, that Pasteur entered more deeply into the train of thought which was to lead him to grapple with Mitscherlich’s problem. “One day” (this is a manuscript note of Pasteur’s) “one day it happened that M. Laurent--studying, if I mistake not, some tungstate of soda, perfectly crystallized and prepared from the directions of another chemist, whose results he was verifying--showed me through the microscope that this salt, apparently very pure, was evidently a mixture of three distinct kinds of crystals, easily recognizable with a little experience of crystalline forms. The lessons of our modest and excellent professor of mineralogy, M. Delafosse, had long since made me love crystallography; so, in order to acquire the habit of using the goniometer, I began to carefully study the formations of a very fine series of combinations, all very easily crystallized, tartaric acid and the tartrates.” He appreciated any favourable influence on his work; we find in the same note: “Another motive urged me to prefer the study of those particular forms. M. de la Provostaye had just published an almost complete work concerning them; this allowed me to compare as I went along my own observations with those, always so precise, of that clever scientist.” Pasteur and Laurent’s work in common was interrupted. Laurent was appointed as Dumas’ assistant at the Sorbonne. Pasteur did not dwell upon his own disappointment, but rejoiced to see honour bestowed upon a man whom he thought worthy of the first rank. Some judges have thought that Laurent, in his introductory lesson, was too eager to expound his own ideas; but is not every believer an apostle? When a mind is full of ideas, it naturally overflows. It is probable that Pasteur in Laurent’s place would have kept his part as an assistant more in the background. He did not give vent to the slightest criticism, but wrote to Chappuis. “Laurent’s lectures are as bold as his writings, and his lessons are making a great sensation amongst chemists.” Whether one of criticism or of approbation, this sensation was a living element of success. In order to answer some insinuations concerning Laurent’s ambition and constant thirst for change, Pasteur proclaimed in his thesis on chemistry how much he had been “enlightened by the kindly advice of a man so distinguished, both by his talent and by his character.” This essay was entitled “_Researches into the saturation capacity of arsenious acid. A study of the arsenites of potash, soda and ammonia._” This, to Pasteur’s mind, was but schoolboy work. He had not yet, he said, enough practice and experience in laboratory work. “In physics,” he wrote to Chappuis, “I shall only present a programme of some researches that I mean to undertake next year, and that I merely indicate in my essay.”