Continuing his journey to Holland, he arrived at Harderwyk at the end of May, and made application for his degree, which he received on the 24th June. His thesis was on intermittent fevers, one of the principal causes of which he maintained to be water impregnated with argillaceous substances. Though he had now accomplished his chief object, he resolved, before returning to Sweden, to make himself known to some of the Dutch literati; and for this purpose proceeded to Leyden, where he hired a lodging. Here he made the acquaintance of Professor Royen, Dr Van Swieten, Lieberkuhn, Gronovius, and several others. By the advice and assistance of the last of these scholars, he published his Systema Naturae, in fourteen folio pages. This little work, containing a compendious classification of the three kingdoms of nature, was very favourably received. Boerhaave, one of the most illustrious physicians that the world has ever seen, was at the same time the most eminent individual in the University of Leyden. Linnaeus was therefore anxious to see him; but finding an interview impracticable in the ordinary way, on account of the great man's constant occupation, he resolved to send him a copy of his treatise, accompanied with a letter. The consequence was an invitation to meet him at his villa near the city, where he had a botanical garden. On entering into conversation with the young foreigner, the venerable professor, astonished at his knowledge, strongly advised him to remain in Holland, where he might be sure of making a fortune; but the other, prevented by poverty from adopting this counsel, was obliged to set out on the following day. Arriving at Amsterdam with a recommendation to Professor Burmann, he was kindly received by that gentleman, who was then occupied with his description of the plants of Ceylon. Linnaeus himself relates the occurrences which took place during this interview: "'Do you wish to see my plants?' asked Burmann, 'which of them would you inspect?' He held out one, and observed, 'It is very rare.' I begged a single flower, which I examined after softening it in my mouth, and pronounced it to be a species of laurus. 'It is not a laurus,' said Burmann. 'But it is,' said I; 'it is the cinnamon-tree.'--'It certainly is the cinnamon,' rejoined Burmann. I then convinced him that this tree was a species of laurus, and so of other plants. At length he said, 'Will you help me with my work on Ceylon, and you shall lodge with myself?'" To this proposal the other assented, delighted with the prospect of at once adding to his reputation and his knowledge.