Stephen devoted himself chiefly to the study of mathematics, Joseph to chemistry; and they were accustomed to form their plans in concert. It appears that they had long contemplated, with philosophical interest, the floating and ascent of clouds in the air, and when they heard of or read Cavendish's theories in regard to _different kinds of air_, it at once struck them that by enclosing some gas lighter than the atmosphere in a bag, a weight might be raised from the earth into the air. The brothers Montgolfier were men of that vigorous stamp who act promptly on receiving their convictions. At once they set about experimenting, and constructed large bags of paper,--the substance which naturally came readiest to their hands, and which appeared to them to be best suited to their purpose. These were filled with hydrogen gas, which raised them to the ceiling; but, owing to the escape of the gas through the pores and cracks of the case, those embryo balloons descended in a few minutes. Instead of varnishing the paper to prevent the escape of the gas, and supposing, erroneously, that the fault lay in the latter, they sought about for a new gas more suitable to the paper. This they found, as they supposed, in the gas which resulted from the combustion of wet straw and wool, which had an upward tendency, they thought, on account of its electrical properties, which caused it to be repelled from the ground. It is scarcely necessary now to point out that the true cause of the upward tendency lay in the rarefaction of the air by the heat of the fire, and that hot air has a tendency to rise because its bulk is greatly increased beyond the same quantity of the surrounding cold air. Although wrong in assigning the cause of the result, they were right in the application of it. While on a visit to Avignon Joseph Montgolfier procured a silk bag having a small opening at its lower end, and a capacity of about fifty cubic feet. Under the orifice some paper was burnt; the air inside was heated and expanded so as to fill the bag, which, when let go, soared rapidly up to the height of seventy or eighty feet, where it remained until the air cooled and allowed it to descend. Thus did the _first_ balloon ascend in the month of November 1782. Delighted with their success, the indefatigable brothers resolved to make further experiment on a larger scale. They procured a quantity of packcloth or coarse linen, formed it into a globe about ninety feet in circumference, lined it with paper, and lighted a fire under it in an iron choffer. This balloon went up with a force which they estimated as equivalent to 500 pounds.