_Answer._ Yes; along the sides and ends near the plates, which are covered with water on the opposite side. At these points the coal is usually “dead” or incandescent, as it remains at too low a temperature to burn. For this reason, in some cases a space of from 8 to 12 inches on each side and still more at the ends of the grates, is made of solid plates, without any openings, and therefore called “_dead-grates_,” so that no cold air can enter at those points. These plates are made sloping downwards from the sides towards the centre of the fire-box, so that the coal which falls on them and is thus coked, can easily be raked towards the middle of the fire. This arrangement of dead plates often improves the combustion and results in greater economy of fuel. The reduction of the area of the openings between the grate-bars can usually be compensated by making the bars narrower or the spaces between them wider. QUESTION 393. _What should be the condition of the coal when it is put on the fire?_ _Answer._ It is true of the coal as well as of the gases that the chemical action between it and the oxygen can only take place when the two are in intimate contact, and therefore the rapidity and completeness of combustion and intensity of heat will be increased by increasing the number of points of contact, or by reducing the size of the fuel. The coal should therefore be broken up, but not so small as to fall between the grate-bars or be carried out of the fire-box by the blast. QUESTION 394. _What amount of air must be admitted to the fire to effect perfect combustion?_ _Answer._ It was stated that average bituminous coal contains about 80 per cent. carbon, 5 per cent. of hydrogen and 15 per cent. of other substances. As a large proportion of the latter are incombustible, we will confine ourselves for the present to the consideration of the combustion of the hydrogen and carbon alone. The hydrogen, as has been explained, unites with oxygen in the proportion by weight of 1 part of the former to 8 parts of the latter, and the product of this union is water, or steam. As 36 parts of air contain only 8 of oxygen, IN ORDER TO BURN THE HYDROGEN IT MUST BE SUPPLIED WITH 36 TIMES ITS WEIGHT OF AIR. In order to burn the carbon perfectly it must, as has been explained, be converted into carbonic dioxide, which consists of 6 parts of carbon and 16 of oxygen; and as air consists of 28 parts of nitrogen to every 8 of oxygen, we must furnish 72 parts of air to every 6 of carbon, or, in other words, CARBON NEEDS 12 TIMES ITS WEIGHT OF AIR FOR ITS PERFECT COMBUSTION.