I think that we have already a sufficient quantity in the bladder for our purpose; we must be careful to stop both the cocks before we separate the bladder from the receiver, lest the gas should escape. --Now I must fix a pipe to the stopper of the bladder, and by dipping its mouth into the soap and water, take up a few drops--then I again turn the cock, and squeeze the bladder in order to force the gas into the soap and water at the mouth of the pipe. (PLATE IX. fig. 2.) EMILY. There is a bubble--but it bursts before it leaves the mouth of the pipe. MRS. B. We must have patience and try again; it is not so easy to blow bubbles by means of a bladder, as simply with the breath. CAROLINE. Perhaps there is not soap enough in the water; I should have had warm water, it would have dissolved the soap better. EMILY. Does not some of the gas escape between the bladder and the pipe? MRS. B. No, they are perfectly air tight; we shall succeed presently, I dare say. CAROLINE. Now a bubble ascends; it moves with the rapidity of a balloon. How beautifully it refracts the light! EMILY. It has burst against the ceiling--you succeed now wonderfully; but why do they all ascend and burst against the ceiling? MRS. B. Hydrogen gas is so much lighter than atmospherical air, that it ascends rapidly with its very light envelope, which is burst by the force with which it strikes the ceiling. Air-balloons are filled with this gas, and if they carried no other weight than their covering, would ascend as rapidly as these bubbles. CAROLINE. Yet their covering must be much heavier than that of these bubbles? MRS. B. Not in proportion to the quantity of gas they contain. I do not know whether you have ever been present at the filling of a large balloon. The apparatus for that purpose is very simple. It consists of a number of vessels, either jars or barrels, in which the materials for the formation of the gas are mixed, each of these being furnished with a tube, and communicating with a long flexible pipe, which conveys the gas into the balloon. EMILY. But the fire-balloons which were first invented, and have been since abandoned, on account of their being so dangerous, were constructed, I suppose, on a different principle. MRS. B. They were filled simply with atmospherical air, considerably rarefied by heat; and the necessity of having a fire underneath the balloon, in order to preserve the rarefaction of the air within it, was the circumstance productive of so much danger. If you are not yet tired of experiments, I have another to show you. It consists in filling soap-bubbles with a mixture of hydrogen and oxygen gases, in the proportions that form water; and afterwards setting fire to them. EMILY. They will detonate, I suppose? MRS. B.