Intelligence and Miscellaneous Articles. S97 



weight of platina, the most delicate balance would not turn with so 

 ligln a load ; besides, they would argue, there are many sources of 

 minute error, which prevent the delicate experiment of ascertaining 

 the supposed no change of weight, with change of heat, being de- 

 cisive; such as change of length of the arm of the balance, change 

 in the specific gravity of the body heated or cooled, and other 

 errors the exact amount of which cannot be ascertained. The same 

 objection may be urged against the apparently decisive conclusion, 

 derived from the fact that when given weights of hydrogen and 

 oxygen are combined by combustion, the weight of the water is 

 equal to the sum of the weights of the elements though very in- 

 tense heat is produced during the combination. It must, I think, 

 be acknowledged that human experiment proves nothing more than 

 that if there be any gravitating force it is extremely small. But 

 nature tries the experiment for us on a scale of magnificence, 

 which we may in vain attempt to imitate, for if heat have weight 

 it must necessarily when in motion have momentum ; and if the 

 velocity of radiant heat from the sun be equal to that of light, 

 some momentum should be discoverable. But granting that the 

 momentum is too small in amount to show itself on the small scale, 

 it would be sure, did it exist at all, to increase the period of revo- 

 lution of the planets. It has been proved that the planets revolve 

 at such distances from the sun, and with such velocities, as that 

 the centrifugal and attractive forces shall be equal, if the later 

 force be inversely as the square of the distance. But if heat had 

 any momentum, its particles, acting upon such large masses as the 

 planets, must produce an evident effect by increasing their distances 

 and periodic times. Now these are not increased ; there can then be 

 no centrifugal force from this cause, heat can have no momentum, 

 and therefore no weight. 



If it be objected to this conclusion that the ratio of attraction 

 has been over-estimated, that it is sufficiently powerful to balance 

 both the tangential force of revolution and the centrifugal force of 

 the momentum of radiant heat ; I answer that this supposition, vio- 

 lent as it is, would not remove the difficulty unless all the planets 

 were of the same size and mass, or unless the sectional area of all 

 were proportionate to the mass. 



The density, or rather rarity, of the resisting medium which has 

 accelerated Encke's comet, has not been ascertained, but it can 

 hardly be such as to counterbalance the supposed centrifugal 

 momentum of heat ; for if the density be uniform, then the velocity 

 of the planets through it should be proportionate to the decrease 

 of the rays of heat, that is, inversely as the square of the distance 

 from the sun. If the density be inversely as the square of the di- 

 stance (supposing the medium analogous to an atmosphere), the ve- 

 locity of the planets through it should be uniform : neither being 

 in accordance with facts. Yours, &c. 



Manchester, Aug. 13, 1836. P. W. Holland. 



