Nothing else; except that the gas must be made to pass, immediately at its formation, through two or three large vessels of water, in which it deposits some other ingredients, and especially water, tar, and oil, which also arise from the distillation of coals. The gas-light apparatus, therefore, consists simply in a large iron vessel, in which the coals are exposed to the heat of a furnace,--some reservoirs of water, in which the gas deposits its impurities,--and tubes that convey it to the desired spot, being propelled with uniform velocity through the tubes by means of a certain degree of pressure which is made upon the reservoir. EMILY. What an admirable contrivance! Do you not think, Mrs. B., that it will soon get into universal use? MRS. B. Most probably, as to the lighting of streets, offices, and public places, as it far surpasses any former invention for that purpose; but as to the interior of private houses, this mode of lighting has not yet been sufficiently tried to know whether it will be found generally desirable, either in regard to economy or convenience. It may, however, be considered as one of the happiest applications of chemistry to the comforts of life; and there is every reason to suppose that it will answer the full extent of public, expectation. I have another experiment to show you with hydrogen gas, which I think will entertain you. Have you ever blown bubbles with soap and water? EMILY. Yes, often, when I was a child; and I used to make them float in the air by blowing them upwards. MRS. B. We shall fill some such bubbles with hydrogen gas, instead of atmospheric air, and you will see with what ease and rapidity they will ascend, without the assistance of blowing, from the lightness of the gas. --Will you mix some soap and water whilst I fill this bladder with the gas contained in the receiver which stands on the shelf in the water-bath? CAROLINE. What is the use of the brass-stopper and turn-cock at the top of the receiver? MRS. B. It is to afford a passage to the gas when required. There is, you see, a similar stop-cock fastened to this bladder, which is made to fit that on the receiver. I screw them one on the other, and now turn the two cocks, to open a communication between the receiver and the bladder; then, by sliding the receiver off the shelf, and gently sinking it into the bath, the water rises in the receiver and forces the gas into the bladder. (PLATE IX. fig. 1.) [Illustration: Plate IX. Vol. I. p. 228 Fig. 1. Apparatus for transferring gases from a Receiver into a bladder. Fig. 2. Apparatus for blowing Soap bubbles.] CAROLINE. Yes, I see the bladder swell as the water rises in the receiver. MRS. B.