And his friends had not forgotten him as he lay there living in his tomb. Especially the learned Scriverius, Vossius, and other professors, were permitted to correspond with him at intervals on literary subjects, the letters being subjected to preliminary inspection. Scriverius sent him many books from his well-stocked library, de Groot's own books and papers having been confiscated by the government. At a somewhat later period the celebrated Orientalist Erpenius sent him from time to time a large chest of books, the precious freight being occasionally renewed and the chest passing to and from Loevestein by way of Gorcum. At this town lived a sister of Erpenius, married to one Daatselaer, a considerable dealer in thread and ribbons, which he exported to England. The house of Daatselaer became a place of constant resort for Madame de Groot as well as the wife of Hoogerbeets, both dames going every few days from the castle across the Waal to Gorcum, to make their various purchases for the use of their forlorn little households in the prison. Madame Daatselaer therefore received and forwarded into Loevestein or into Holland many parcels and boxes, besides attending to the periodical transmission of the mighty chest of books. Professor Vossius was then publishing a new edition of the tragedies of Seneca, and at his request Grotius enriched that work, from his prison, with valuable notes. He employed himself also in translating the moral sentences extracted by Stobaeus from the Greek tragedies; drawing consolation from the ethics and philosophy of the ancient dramatists, whom he had always admired, especially the tragedies of Euripides; he formed a complete moral anthology from that poet and from the works of Sophocles, Menander, and others, which he translated into fluent Dutch verse. Becoming more and more interested in the subject, he executed a masterly rhymed translation of the 'Theban Brothers' of Euripides, thus seeking distraction from his own tragic doom in the portraiture of antique, distant, and heroic sorrow. Turning again to legal science, he completed an Introduction to the Jurisprudence of Holland, a work which as soon as published became thenceforward a text-book and an oracle in the law courts and the high schools of the country. Not forgetting theology, he composed for the use of the humbler classes, especially for sailors, in whose lot, so exposed to danger and temptation, be ever took deep interest, a work on the proofs of Christianity in easy and familiar rhyme--a book of gold, as it was called at once, which became rapidly popular with those for whom it was designed.