262 Scientific Intelligence. 
no cause for undulatory motion when a body is in a state of rest, or in a 
state of equilibrium with the medium by which it is surrounded. If a 
bell or a string is to be sounded an external force must be applied; and 
this is the cause of the soun 
If the vibratory motion of a string could take place without any 
resistance, it would vibrate for all time; but in this case no sound could 
be produced, because sound is essentially “hi propagation of motion ; and 
in the same de egree as the string communicates its vibrations to the sur- 
rounding and resisting medium its own motion becomes weaker and 
weaker, until it at last sinks into a state of rest. 
de e sun has often and appropriately been compared to an peer 
ing bell. But by what means is the power of this body kept up 
in Si dndiciniahed force so as to enable him to send forth his rays info the 
universe in such a grand and magnificent manner ? at are the causes 
which counteract or prevent his saa and thus save the planetary 
+ ee darkness and deadly co 
eavored to approach “ he Hae secret,” as Sir Wm. Herschel 
calls this ateaiion, by the assumption that the rays of the sun, being 
themselves es perfectly cold, merely cause the “ mene: of heat, supposed 
to be contained in bodies, to pass from a state of rest into a state of 
ear and that in oer to send forth such cold rays vike sun need not 
ot body, so that, in spite of the infinite oe gue of light, the. 
of.- 
¢eooling of the sun was a matter not to be thou 
It is A pete that nothing is gained by such an explanation for, not to 
thetical “substance” of heat, assumed to be at one 
time ‘e rest and at another time in motion, now cold and then hot, it is 
thing, grind different it may be in other respects, can always be 
referred two categories: either it consists of some material 
Pa in hema proces, or of some sort of mechanical work. — 
d with considerable chemical ame eee : 
Pe sey law of nature, which knows of no exception, is the follow- 
fet order to obtain heat something must be ee ks! some- 
