The giant holders are reservoirs from which gas is constantly being taken and the quantity on storage constantly replenished, as the ordinary gas plant never ceases manufacturing its product. There is little or no danger of an interruption of the supply by reason of accident, as gas plants are always equipped with duplicate apparatus for emergencies.] When Illuminating Gas Was Discovered. The first practical demonstration of the value of gas made from coal for lighting was made by a Scotchman--Robert Murdock--who in 1797, after some years of experimenting, fitted up an apparatus in the workshop of Boulton and Watt, in Birmingham, England, which successfully lighted a portion of that establishment. The advantages of this kind of lighting were so apparent that its use was rapidly extended, although in many instances the people were afraid of it. For a time this kind of lighting was confined to street lights. One of the first great structures to be lighted by gas was Westminster Bridge in London, and great crowds gathered to watch the burning jets nightly. It was difficult to remove from the minds of the people the belief that the gas-pipes were filled with fire and the jets were only openings through which the flame in the pipes escaped. People sometimes touched the pipes expecting to find them hot, and when the pipes were put in buildings they made sure that they were placed several feet from the walls lest the fire in them set fire to the buildings. The use of illuminating gas for lighting private houses developed quite slowly because of this fear of the fire in the gas-pipes. This was not entirely unwarranted, however, because at first the plumbers did not know, as they do now, how to prevent leakage of gas from the pipes. The methods of joining the pipes were oftentimes imperfect and, not realizing the dangers which would follow leaks, causing explosions, the workmen were often careless in installing the pipes. The first American house in which gas was used for lighting was the home of David Mellville at Newport, R. I. Baltimore, Maryland, was the first American city to use gas for lighting. It was introduced there in 1817. How Does Gas Get Into the Gas Jet? If you hold a cool drinking glass over a burning gas jet for a moment, a film of moisture will form on the inside of the glass and remain until the tumbler becomes warm, and then disappear. Now, then, you will remember that water is a mixture of oxygen and hydrogen, and that when hydrogen is burned in the air, water is formed. It is also true that whenever water is formed by burning anything, hydrogen is present in it. You see, therefore, that the gas used for lighting purposes must contain hydrogen.