1825.] Mr. Powell on Terrestrial Light and Heat. 367 



derive its projectile impulse from the sole operation of its pecu- 

 liar elasticity while in the state of heat." " Its motion," he then 

 shows, " is exactly similar to that with which an expansive 

 fluid will rush into a vacuum." He concludes this profound 

 investigation by remarking, that " We are forced to suppose 

 that when bodies discharge light, they are thrown into a sort of 

 convulsive state, having their adhesive attraction to it affected 

 by momentary intervals of suspension, during which fits, the 

 luminous particles, being set free, are projected by their own 

 intrinsic repulsione. Without admitting this hypothesis, it 

 seems impossible to explain the equality of motion which belongs 

 to every species of light, however variously combined with dif- 

 ferent bodies as constituting heat, it is emitted from them all 

 with the same rapidity ; and such we have seen is the remarka- 

 ble property of an expansive fluid when liberated." (P. 177.) 

 He then proceeds by a highly curious computation to calculate 

 the elasticity of light, the weight of combined light on heat, the 

 waste of luminous matter in the sun, and other points coiniected 

 with these. 



In thus explaining the projection of light from bodies, its 

 existence as an expansive and elastic fluid is easily admissible, 

 and appears conformable with all we know of its properties. 

 But the hypothesis just quoted respecting the state into which 

 budies must be brought in order to discharge it does not seem 

 susceptible of experimental proof. 



Admitting the reasoning from which the phenomena of its 

 emission are explained on the supposition of its being an elastic 

 fluid, we may ask is it necessary to suppose in order to its being 

 thus emitted that it is identical with heat? Should we not avoid 

 the part of the hypothesis last alluded to, if we supposed the 

 light to exist in combination with the solid substance in the same 

 ■vvay as gaseous fluids are known to exist in combination with 

 such substances, and that by the operation of the heat which is 

 employed in raising the body to the temperature of luminosity, 

 the light before in combination is made to assume its elastic 

 fluid state, and then is projected from the body according to the 

 mathematical principles which Prof. L. has just before laid 

 down. 



From this review, we shall be prepared to perceive how the 

 circumstance before adverted to presents a serious difficulty to 

 the hypothesis. If the heat of a body be converted into light, 

 owing to the action of the causes here explained, we may ask 

 how then it can happen that only ;)a// of the whole quantity of 

 heat combined with the body is tluis changed into light? or why 

 the increase of elasticity only takes place in part of the combined 

 fluid, and not in the whole; for the unaKered portion still con- 

 tinues to be radiated as heat, but is neither converted into light, 



