This danger of fire was done away with in the method of filling with hydrogen gas. The balloon, suspended from aloft as before, was connected by hose-pipes with a number of casks containing iron or zinc filings upon which dilute sulphuric acid was poured. The effect of mixing these substances together is to set up a brisk chemical action, in the course of which hydrogen gas is given off. In this case the hydrogen thus liberated came through the pipes and filled the balloon. The great disadvantages of this method of filling--which, it may here be mentioned, is occasionally employed at the present day--are the long time it occupies, the great labour entailed, and the enormous expense. [Illustration: FILLING A HYDROGEN GAS BALLOON.] It is said that when Roberts and Charles returned from their adventurous voyage they were immediately arrested and thrown into prison by order of the King, who considered it his duty to put a stop to his subjects risking their lives in such dangerous enterprises. Public opinion was too strong for him, however, and the two heroes were quickly released, and Charles was rewarded by a pension of £200 a year for life. This newly discovered art of sailing the heavens had indeed fired popular imagination to an extraordinary degree. Probably no invention has ever aroused greater enthusiasm. Not only all France but all the civilised world went wild with excitement for the time. Most extravagant statements were made and written. A new kingdom, it was declared, had been given to mankind to conquer; voyages might be made to the moon and stars, and now it would even be possible to take Heaven itself by storm! Ascent after ascent took place with the “Montgolfier” and the “Charlier,” both in France and in other countries; nor was it long before the balloon made its appearance in England. In August of the next summer (1784) a Mr Tytler of Edinburgh made some short voyages in a hot-air balloon of his own manufacture, and in the following month a much more adventurous attempt was successfully carried out in London by a young Italian of the name of Vincent Lunardi.