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McAllister, Optician, 49 Nassau St., New Yoke:. If the Gas comes over too violently, remove the lamp and replace it when the flow of gas becomes more moderate, and keep it under the retort until the bubbles cea.se to come over. You then turn the cock at the. Bag—detach the Bag from the Wash Bottle. Next remove the metal pipe from the Wash Bottle—and, last of all, remove.the Spirit Lamp. When the Ketort has cooled, wash out the residuum in the Retort, and stand the Retort upside down, that it may be entirely dry before it is used again, as it is absolutely necessary that there shall be no moisture in the Ketort while the Gas is being made. With good chemicals the Bag can be filled with Gas in about twenty minutes. Failures often occur from using impure chemicals. To test them, melt a small quantity of the Chlorate of Potash, in an iron spoon, over a Spirit Lamp, and stir into it with an iron wire some of the Black Oxide of Manpfauese. If the materials are not good an explosion will take place, and a whitish mass with red spots in it will be left in the spoon,—if they are pure, there will be no explosion, and the melted mix- ture will soon dry up, leaving a dark gray residuum. For price of Chemicals, etc., see price list on page 48. PRESSURE BOARDS—All the Oxy-Hydrogen and Oxy-Calcium Stereopticons and Lanterns are furnished comylete, with every necessary except the Pressure Boards for the Gas Bags; and these can be made by any carpenter at a less cost than the express charge on them would amount to. GAS BAGS.—The India Rubber Gas Bags will contain enough Gas for an evening's exhibition, lasting from one hour aud-a-half to two hours, according as a greater or less pressure* is placed upon the bag. The greater the pressure, the more gas will be consumed in a given time. Extra large Gas Bags, or two Gas Bags can be furnished at an extra cost. No. 421. McAllister's Improved Adjustable Oxy-Calcium Jet This Jet is very simple in its construction. ai)d far superior to the complicated Oxy-Calcium Jets formerly used. It is mounted on a walnut base, 4 inches wide by 12 inches long. Haa stop-cock to control the flow of gas, elevating and centering screw, etc. Can be readily applied to any lantern $8 00 No. 422. Oxygen Apparatus, consisting of Retort, with Spirit Lamp, Purifier, India Rubber Gas Bag, with Stop-Cook. $45 00