THE SOHECE OF HEAT TN THE SUN. 
147 
produced_, that the body^ if metal^ will become incandescent ; 
if wood^ it will take fire. ISTow^ this is regarded as the conver- 
sion of motion into another form of force — heat ; and hence the 
hypothesis that heat is a mode of motion. There are several 
modes of motion^ but in all of them the relation of heat to the 
disturbance and the resistance obeys the same law. The law 
of equivalents is unvarying. The same quantity of the electric 
fluid which passes through a good conductor such as copper 
wire^ without producing any sensible increase of temperature^ 
is developed as heat and light in passing through a had con- 
ductor such as platinum, the obstruction offered to its motion 
determining the result. Chemical action^ again_, produces 
motion amongst the combining particles^ and according* to the 
rapidity of combination, so is the intensity of the calorific or 
luminous effects produced. It is not possible to pursue this 
subject, notwithstanding its interest, any further in this place,^ 
since other matters claim our close attention.* The law of 
equivalents to which reference has been made, prevails in all 
the operations of nature. Not merety is the equivalent of heat 
and light produced, determined exactly by the amount of motion 
established and converted into force, but an exact quantity (an 
equivalent) of matter undergoes a change of form in the pro- 
duction of any physical effect. In the ordinary processes for 
producing heat or light, carbon is the element upon which we 
depend. Whether we employ coal or wood in our common 
fires or our furnaces ; whether we use candles, animal or vege- 
table oil, or paraffin, or gas, as the illuminating agent, the 
quantity of carbon present regulates the result. For the 
moment, the hydrogen in combination is neglected, as tending 
to complicate the explanation ; the result is, however, pre- 
cisely the same. ' The solid carbon in any of the substances 
named is brought in contact with a burning body, and it 
begins to burn — that is, it seizes oxygen from the air, and in 
accordance with the energy with which this seizure is made, 
so is the intensity of the result. The quantity of heat or of 
light given out is measured by the rapidity with which oxygen 
enters into combination and forms carbonic acid, or an oxide 
of carbon. To take another illustration, the metal magnesium 
is drawn into wire, and it does not oxidize (tarnish) readily 
under ordinary circumstances ; but ignite it in a flame, and it 
seizes oxygen with such avidity, that an intense heat is gene- 
rated, and the pure white particles of magnesia formed become 
so hot that they glow with the utmost intensity of light. In 
either of the examples given, the degree of excitement pro- 
The reader who is desirous of examining fully the whole question, is 
referred to “ Heat considered as a Mode of Motion,” ly John Tyndall, F.E.S. 
L 2 
