It is stated, that about this period he made an important discovery relative to the formation of pearls in the river-mussel (_Unio margaritifera_), a shell of common occurrence in the northern parts of Europe as well as in our own country, and from which are obtained all our indigenous pearls, which not many years ago were held in considerable estimation. By injuring the shell, probably by means of puncture or perforation, it is supposed that he succeeded in causing a deposition of the pearly matter, so that one might procure a certain quantity at pleasure. The precise method, however, is still uncertain, nor is it believed to have been generally successful; at all events the secret has been entirely lost. At this period, says Linnaeus, botany was cultivated at Upsal with unparalleled ardour. Frequent excursions were made for the purpose of collecting plants, insects, and birds. Every Wednesday and Saturday herbarizations took place, which continued from dawn to night. The pupils, having their hats covered with flowers, returned to the town, and preceded by musical instruments accompanied their professor to the garden. But amid all this success he was harassed by the malice of his enemies. A decree of the senate appeared, which prevented any native of Sweden from publishing a work in a foreign country. This was evidently directed against him alone, for, as he says, it could apply to no other person. In a fit of bad humour he flung his pen from him, and swore that he would never write another book. At this period also a person named Fick endeavoured, by disgraceful calumnies, to injure him in the esteem of his fellow-citizens. This conduct he felt so much the more severely, because the slanderer was one of his familiar friends, which was also the case with respect to Halenius, who openly censured one of his dissertations, although he had approved of it before it was sent to press. About the same time he received a letter from Haerlem, which he says nearly cost him his life, and prevented him from sleeping for two months. The purport of this communication has not been disclosed; but, surely, if he had not placed too much value on the opinion of the world, he would have allowed the malice of his enemies to vent itself in impotent rage. His self-love, however, was soon gratified by the arrival of a pupil from Paris, the first who had come to him from a foreign country, and by the presence of several persons of distinction at his excursions. This year he had a hundred and forty students at his lectures.