The essential features of all gas lamps used under these conditions are shown in Fig. 192, which represents the common bracket type of lamp. The gas-cock _C_, connects the lamp with the gas supply _G_. The gas escapes into the Bunsen tube, through an opening in the tip _P_, which is so constructed that the amount of gas may be varied to suit the required conditions. The brass screw nut _N_ may be raised or lowered and thus increase or diminish the amount of escaping gas by reason of the position of the pin _P_. If the nut is screwed completely down the pin closes the opening and the gas is entirely shut off. When the lamp is put in place, the burner is adjusted to admit the proper amount of gas and so long as the quality of the gas remains the same, no further adjustment will be necessary. Any change to a richer or poorer gas will, however, require an adjustment of the burner to suit the mantle. The amount of gas admitted is only that which will produce complete combustion in the mantle when combined with the required amount of air. Each burner must, therefore, be designed for the mantle in use. [Illustration: FIG. 192.--Mantle gas lamp showing details of Bunsen tube.] As the gas leaves the opening above the pin _P_, it enters the mixing chamber of the Bunsen tube and air is drawn at the openings _A_-_A_. The mixture of the gas and air is accomplished in the tube leading to the mantle _M_, where it is burned. In all lamps of this kind, there is a wire screen placed relatively as _S_, the object of which is to prevent the mixture in the tube from exploding--in case of low pressure--and thus cause the gas to ignite and burn at the point of entrance to the tube. At any time the pressure is insufficient to send a steady flow of gas into the tube, the flame may “flash back” and ignite the gas at the point of entrance where it will continue to burn. If, however, the screen is interposed between the gas supply and the burner, the flame of explosion will not pass the screen. In lighting the lamp, the gas is turned on and a lighted match is held under the mantle, the explosive mixture of gas and air fills the mantle and escapes into the globe, in which it is usually inclosed. As soon as ignition takes place the gas outside the mantle explodes with the effect that is startling but not necessarily dangerous. The escaping gas continues to burn and heats the mantle to incandescence. The amount of escaping gas is regulated by turning the gas-cock to produce the greatest brilliance with the least flame outside the mantle. When used for household illumination, the intensity of the light is such as to be objectionable, when used directly; but when surrounded by an opal glass globe to diffuse the light, this is a highly satisfactory and economical means of lighting.