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?_
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.