A large portion of Leopold Mozart's letters is occupied with masses to be offered up for the health, &c.; and during his sojourn in the Five-fields, Chelsea, he appears to have been in considerable hope that he had converted a Mr. Sipruntini (a Dutch Jew, and a fine violoncello player), to Catholicism. After dedicating a set of sonatas to the queen, and experiencing great patronage from the nobility, Mozart, with his father and sister, in July, 1765, crossed over into the Netherlands. At the Hague, a fever attacked both children, and had nearly cost the daughter her life. On their recovery, they played before the Prince of Orange, and Wolfgang composed some variations on a national air, which was, just then, sung, piped, and whistled throughout the streets of Holland. The organist of the cathedral in Haerlem waited upon the Mozarts, and invited the son to try his instrument, which he did the next morning. Mozart senior describes the organ as a magnificent one, of sixty-eight stops, and built wholly of metal, "as wood would not endure the dampness of the Dutch atmosphere." Upon the return of the family to Salzburg, Mozart enjoyed a year of quiet and uninterrupted study in the higher walks of composition. Besides applying to the old masters, he was indefatigable in perusing the works of Emanuel Bach, Hasse, Handel, and Eberlin, and by the diligent performance of these authors, he acquired extraordinary brilliancy and power in the left hand. On the 11th of September, 1767, the whole family proceeded on their way to Vienna; but as the small pox was raging there, they went to Ollmütz instead, where both the children caught that disorder. At Vienna, Mozart wrote his first opera, by desire of the emperor. Though the singers extolled their parts to the skies, in presence of Leopold Mozart, they formed in secret a cabal against the work, and it was never performed. The Italian singers and composers who were established in this capital did not like to find themselves surpassed in knowledge and skill by a boy of twelve years old, and they therefore not only charged the composition with a want of dramatic effect, but they even went so far as to say, that he had not scored it himself. To counteract such calumnies, Leopold Mozart often obliged his son to put the orchestral parts to his compositions in the presence of spectators, which he did with wonderful celerity before Metastasio, Hasse, the Duke of Braganza, and others. The injurious opinion of the nobility, which these people hoped to excite against the young musician, had no success; for he composed a Mass--an Offertorium--and a Trumpet Concerto for a Boy--which were performed before the whole court, and at which he himself presided and beat the time. The year 1769 was employed by Wolfgang in studying the Italian language, and in the practice of composition; and at this time he was appointed concert master to the court of Salzburg.