He was then weary of his pursuits, and it is said that his _Mécanique Analytique_, which he had sent from Berlin to be printed in Paris, lay unopened by himself for more than two years after its publication in 1788. He employed himself in the study of ecclesiastical and other history, of medicine, botany, and metaphysics. When the discoveries of the chemists changed the theory and notation of their science, or rather created a science where none existed before, he threw himself upon the new study with avidity, and declared that they had made it easy; _as easy as algebra_.
In 1792, being then fifty-six years of age, he married Mlle. Lemonnier, daughter of the astronomer of that name, and daughter, grand-daughter, and niece of members of the Academy of Sciences. This lady well deserves honourable mention in every memoir of Lagrange, for the affectionate care which she took of his declining years.
When, after the subversion of the monarchy, a commission was appointed to examine into the system of weights and measures, Lagrange was placed at its head. In this post he continued, not being included in the _purification_, which three months after its formation, deprived the commission of the services of Laplace, Coulomb, Brisson, Borda, and Delambre. He took no part in politics, and appears to have given no offence to any party; hence, when the government of Robespierre commanded all foreigners to quit France, an exception was made in his favour by the committee of public safety. All his friends had advised him to retire from the country; and the fate of Lavoisier and Bailly was sufficient to show that scientific talents of the most useful character were no protection. He now regretted that he had not followed their advice, and even meditated returning to Berlin. He did not, however, put this scheme in execution; and as the Normal and Polytechnic Schools were successively founded, he was appointed to professorships in both. His _Leçons_, delivered to the former institution, appear in their published series, and among them we find the _Leçons sur la Théorie des Fonctions_, which has since appeared as a separate work.
It is almost needless to say, so well as the public know how science was encouraged under the Consulate and the Empire, that Lagrange received from Napoleon every possible respect and distinction. The titles of senator, count of the empire, grand cordon of the legion of honour, &c. were given to him. It is also gratifying to be able to add that his abstinence from political engagements has left his memory unstained by such imputations as, we know not how justly, rest upon that of Laplace. We might have omitted to state that he belonged to all the scientific academies of Europe; but that it is necessary, for the sake of the scientific reputation of this country, to correct an inadvertence into which the able author of the ‘Life of Lagrange,’ in the _Biographie Universelle_, appears to have fallen. He states that Lagrange was not a member of the Royal Society of London[9]. The fact is, that he was elected in 1798, and his name continued on the list of foreign members all the remainder of his life.