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THE POPULAR SCIENCE MONTHLY. -SUPPLEMENT. 



among British mathematical physicists, whose 

 opinions I had the opportunity of learning from 

 themselves, were disposed to concur with Mr. 

 Crookes in regarding the repulsion of the heated 

 end of the pith-bar as an immediate effect of ra- 

 diant energy ; dwelling especially on the fact that 

 the repulsion was stronger in proportion to the 

 completeness of the vacuum, from which it seemed 

 fair to infer that it would be most strongly mani- 

 fested in a perfect vacuum (if such could by possi- 

 bility be obtained), in consequence of the entire 

 removal of mechanical resistance (save the fric- 

 tion of the pivot) to the rotation of the mill. 



Now, since our belief in Newton's " First Law 

 of Motion " has no other experiential basis than 

 the fact that, the more completely we can elimi- 

 nate friction and the resistance of the air, the 

 longer is the persistence of motion in a body once 

 put in movement, provided that no opposing force 

 be brought to bear upon it, this argument for the 

 directness of radiant repulsion seemed alike val- 

 id and cogent. We shall presently see, however, 

 that it proves fallacious when brought to an ex- 

 perimental test of greater delicacy. 



The doctrine propounded by Mr. Crookes was 

 first explicitly called in question in a communi- 

 cation made to the Royal Society on the 18th of 

 June, 1874, by Prof. Osborne Reynolds ; who 

 maintained that, on the kinetic theory of gases 

 (which represents any gaseous substance as con- 

 sisting of molecules constantly in motion at great 

 velocities), the effect on the torsion-balance is 

 really due to alternate evaporation of vapor from, 

 and its condensation at, the surface of the pith ; 

 evaporation producing a reactionary force equiv- 

 alent to an increase of pressure on the heated 

 surface, while condensation must be attended with 

 a force equivalent to a diminution of pressure 

 over the cooling surface. Thus, when the heat 

 radiated from the lamp falls on the pith, its tem- 

 perature will rise, and any moisture on it will be- 

 gin to evaporate, thus generating a mechanical 

 force which will drive the pith from the lamp. 

 Conversely, when a piece of ice is brought near, 

 the temperature of the pith will be reduced, caus- 

 ing a condensation of vapor which will cause the 

 pith to move toward the ice. When the two 

 arms of the pith-bar are unequally exposed to 

 heat, the evaporation will be greatest in that 

 which is nearest the lamp ; and this is driven 

 away, therefore, until the force on the other arm 

 becomes equal, after which the bar will come to 

 rest, unless the momentum it has acquired in 

 swinging carries it farther. 



In a subsequent communication (March 23, 



187G) Prof. 0. Reynolds applied a similar doctrine 

 to the continuous rotation of the " light-mill • " 

 maintaining, from theoretical considerations, the 

 existence of reactionary forces, or " heat-reac- 

 tions," whenever heat is communicated from a 

 surface to a gas, and vice versa; and showing 

 that there is enough residual air in the best ex- 

 hausted globe to enable an amount of force to be 

 thus developed, which is sufficient to keep up the 

 rotation of its contained mill. This explanation 

 obviously implies the existence of a reactionary 

 force, communicated by the intervening gas, be- 

 tween the disks of the mill and the inclosing 

 glass ; and the existence of such a reaction was 

 experimentally proved by an ingenious arrange- 

 ment first devised by Dr. Schuster, and subse- 

 quently improved on by Mr. Crookes. A radi- 

 ometer, on whose arms a magnet is fixed, is 

 floated in a vessel of water, round which four 

 candles are fixed, so as to keep the mill in rota- 

 tion. When a powerful magnet is brought near 

 the outside of the globe, the arms immediately 

 stop ; but at the same time the globe begins to 

 revolve slowly in the opposite direction, and con- 

 tinues to do so as long as the candles burn, and 

 the arms remain fixed by the magnet. When the 

 magnet is removed, the "mill" begins to rotate 

 in its original direction, and the glass envelope 

 quickly comes to rest. 



Now, as the existence of a reactionary force, 

 which seems unmistakably indicated by this ex- 

 periment, cannot be accounted for on the suppo- 

 sition that the " mill " is driven round by the im- 

 mediate mechanical impulse of radiation, while it 

 is exactly what would be anticipated if the radi- 

 ant energy acts calorifically on the residual gas, 

 a very strong support is obviously afforded to the 

 latter interpretation. And hence, although phys- 

 icists may differ as to the precise manner in 

 which the unequal heating of the disks produces 

 the movement, 1 there is now, I believe, a very gen- 

 eral accordance in the conclusion that this is the 

 real modus operandi of the radiant energy ; so 

 that, instead of either a " new force," or a " new 

 mode of force," we have simply a well-known 

 mode of force acting under peculiar conditions. 



This conclusion derives very striking confir- 

 mation from two of Mr. Crookes's more recent 

 experiments, which seem to possess a crucial 

 value. Having still further improved his " Sprcn- 

 gel pump," he has been able to carry the exhaus- 

 tion of his globe to a yet greater degree than be- 

 fore, so that its internal condition more nearly 



1 See Mr. Johnstone Stoney, in Philosophical Maga- 

 zine, April, 1876. 



