He now began to read publick lectures with great applause, and was prevailed upon, by his audience, to enlarge his original design, and instruct them in chymistry. This he undertook, not only to the great advantage of his pupils, but to the great improvement of the art itself, which had, hitherto, been treated only in a confused and irregular manner, and was little more than a history of particular experiments, not reduced to certain principles, nor connected one with another: this vast chaos he reduced to order, and made that clear and easy, which was before, to the last degree, difficult and obscure. His reputation now began to bear some proportion to his merit, and extended itself to distant universities; so that, in 1703, the professorship of physick being vacant at Groningen, he was invited thither; but he refused to leave Leyden, and chose to continue his present course of life. This invitation and refusal being related to the governours of the university of Leyden, they had so grateful a sense of his regard for them, that they immediately voted an honorary increase of his salary, and promised him the first professorship that should be vacant. On this occasion he pronounced an oration upon the use of mechanicks in the science of physick, in which he endeavoured to recommend a rational and mathematical inquiry into the causes of diseases, and the structure of bodies; and to show the follies and weaknesses of the jargon introduced by Paracelsus, Helmont, and other chymical enthusiasts, who have obtruded upon the world the most airy dreams, and, instead of enlightening their readers with explications of nature, have darkened the plainest appearances, and bewildered mankind in errour and obscurity. Boerhaave had now for nine years read physical lectures, but without the title or dignity of a professor, when, by the death of professor Hotten, the professorship of physick and botany fell to him of course. On this occasion he asserted the simplicity and facility of the science of physick, in opposition to those that think obscurity contributes to the dignity of learning, and that to be admired it is necessary not to be understood. His profession of botany made it part of his duty to superintend the physical garden, which improved so much by the immense number of new plants which he procured, that it was enlarged to twice its original extent. In 1714, he was deservedly advanced to the highest dignities of the university, and, in the same year, made physician of St. Augustin's hospital in Leyden, into which the students are admitted twice a week, to learn the practice of physick. This was of equal advantage to the sick and to the students, for the success of his practice was the best demonstration of the soundness of his principles.