93. How many kinds of combustion are there?

There are three, viz., slow oxydation, when little or no light is evolved; a more rapid combination, when the heat is so great as to become luminous; and a still more energetic action, when it bursts into flame.94. Why does phosphorous look luminous?

Because it is undergoing slow combustion.95. Why do decayed wood, and putrifying fish, look luminous?

Because they are undergoing slow combustion. In these cases the heat and light evolved are at no one time very considerable. But the total amount of heat, and probably of light, generated through the lengthy period of this slow oxydation, amounts to exactly the same as would be evolved during the most rapid combustion of the same substances.96. What is flame?

It is gaseous matter burning at a very high temperature.97. Why, when we put fresh coals upon a fire, do we hear the gas escaping from the coals without taking fire?

Because, the fire being slow, the temperature is not high enough to ignite the gas.

"I will praise thee, O Lord, with my whole heart; I will show forth thy marvellous work."—Psalm ix.

98. What is the gas which escapes from the coals?

Carburetted hydrogen.99. Why, if we light a piece of paper, and lay it where the gas is escaping from the coals, will it burst into flame?

Because the lighted paper gives a heat sufficient to ignite the gas; and because also hydrogen requires the contact of flame to ignite it.100. Why, when the coals have become heated, will the hydrogen burst into flame?

Because the carbon of the coals, and the oxygen of the air, have begun to combine, and have greatly increased the heat, and have produced a rapid combustion, so nearly allied to flame, that it ignites the hydrogen.101. What temperature is required to produce flame?

That depends upon the nature of the combustible you desire to burn. Finely divided phosphorous and phosphorated hydrogen will take fire at a temperature of 60 deg. or 70 deg.; solid phosphorous at 140 deg.; sulphur at 500 deg.; hydrogen and carbonic oxide at 1,000 deg. (red heat); coal gas, ether, turpentine, alcohol, tallow, and wood, at about 2,000 deg. (incipient white heat). When once inflamed they will continue to burn, and will maintain a very high temperature.102. What is smoke?

Smoke consists of small particles of carbon of hydrogen gas, and other volatile matters, which are driven off by heat and carried up the chimney.103. Is it not a waste of fuel to allow this matter to escape?

It is, as it might all be burnt up by better management.104. How may the waste be avoided?

By putting on only a little coals at a time, so that the heat of the fire shall be sufficient to consume these volatile matters as they escape.

"And the strong shall be as tow, and the maker of it as a spark, and they shall both burn together, and none shall quench them."—Isaiah i.

105. Why is there so little smoke when the fire is red?

Because the hydrogen and the volatile parts of the coal have already been driven off and consumed, and the combustion that continues is principally caused by the carbon of the coals, and the oxygen of the air.106. Will carbon, burnt in oxygen, produce flame and smoke?

It burns brightly, but it produces neither flame nor smoke.107. Why do not charcoal and coke fires give flame?

Because the hydrogen has been driven off by the processes by which charcoal and coke are made.108. What is a conductor of heat?

A conductor of heat is any substance through which heat is readily transmitted.109. What is a non-conductor of heat?

A non-conductor is any substance through which heat will not pass readily.110. Name a few good conductors.

Gold, silver, copper, platinum, iron, zinc, tin, stone, and all dense solid bodies.111. Name a few non-conductors.

Fur, wool, down, wood, cotton, paper, and all substances of a spongy or porous texture.112. How is heat transmitted from one body to another?

By Conduction, Radiation, Reflection, Absorption and Convection.113. What is the Conduction of heat?

It is the communication of heat from one body to another by contact. If I lay a penny piece upon the hob, it becomes hot by conduction.114. What is the Radiation of heat?

The transmission of heat by a series of rays. If I hold my hand before the fire, the rays of heat fall upon it, and my hand receives the heat through radiation.

"Sing praises to the Lord, which dwelleth in Zion, declare among the people his doings."—Psalm ix.

115. What is the Reflection of heat?

The reflection of heat is the throwing back of its rays towards the direction whence they came. In a Dutch oven the rays of heat pass from the fire to the oven, and are reflected back again by the bright surface of the tin. There is, therefore, considerable economy of heat in ovens, and other cooking utensils constructed upon this plan.116. What is the Absorption of heat?

The absorption of heat is the taking of it up by the body to which it is transmitted or conducted. Heat was conveyed to my hand by radiation, and taken up by my hand by absorption.117. What is the Convection of heat?

The convection of heat is the transmission of it through a body or a number of bodies, or particles of bodies, by those substances which first received it; as when hot water rises from the bottom of a kettle and imparts heat to the cold water lying above it.