1. _Arc furnaces._ In the one type the source of heat is an electric arc formed between carbon electrodes separated a little from each other, as shown in Fig. 67. The substance to be heated is placed in a vessel, usually a graphite crucible, just below the arc. The electrodes and crucible are surrounded by materials which fuse with great difficulty, such as magnesium oxide, the walls of the furnace being so shaped as to reflect the heat downwards upon the contents of the crucible. [Illustration: Fig. 68] 2. _Resistance furnaces._ In the other type of furnace the heat is generated by the resistance offered to the current in its passage through the furnace. In its simplest form it may be represented by Fig. 68. The furnace is merely a rectangular box built up of loose bricks. The electrodes E, each consisting of a bundle of carbon rods, are introduced through the sides of the furnace. The materials to be heated, C, are filled into the furnace up to the electrodes, and a layer of broken coke is arranged so as to extend from one electrode to the other. More of the charge is then placed on top of the coke. In passing through the broken coke the electrical current encounters great resistance. This generates great heat, and the charge surrounding the coke is brought to a very high temperature. The advantage of this type of furnace is that the temperature can be regulated to any desired intensity. EXERCISES 1. Why does charcoal usually burn with no flame? How do you account for the flame sometimes observed when it burns? 2. How do you account for the fact that a candle burns with a flame? 3. What two properties must the mantle used in the Welsbach lamp possess? 4. (a) In what respects does the use of the Welsbach mantle resemble that of lime in the calcium light? (b) If the mantle were made of carbon, would it serve the same purpose? 5. Would anthracite coal be suitable for the manufacture of coal gas? 6. How could you prove the formation of carbon dioxide and water in the combustion of illuminating gases? 7. Suggest a probable way in which natural gas has been formed. 8. Coal frequently contains a sulphide of iron. (a) What two sulphur compounds are likely to be formed when gas is made from such coal? (b) Suggest some suitable method for the removal of these compounds. 9. Why does the use of the bellows on the blacksmith's forge cause a more intense heat? 10. What volume of oxygen is necessary to burn 100 l. of marsh gas and what volume of carbon dioxide would be formed, all of the gases being measured under standard conditions? 11. Suppose a cubic meter of Pennsylvania natural gas, measured under standard conditions, were to be burned. How much water by weight would result? CHAPTER XIX MOLECULAR WEIGHTS, ATOMIC WEIGHTS, FORMULAS