Reason Why — Chapter 6
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.
[Verse: "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.
[Verse: "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_.
[Verse: "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?
_
