July 2, 1874 I 



NATURE 



175 



It has been noticed by several philosophers, and particularly 

 by Mr. Crookcs, that, under certain circumstances, hot bodies 

 appear to repel and cold ones to attract o'.her bodies. It is my 

 object in this paper to point out and to describe experiments to 

 prove that these efiects are the results of evaporation and con- 

 densation ; and tliat they are "Nahiable as evidence of the truth 

 of the kinetic theory of gas, viz. that yas consists of separate 

 molecules moving at great velocities. 



The experiments of which the explanation will be given were 

 as follows : — 



A light stem of glass, with pith-balls on its ends, was suspended 

 by a silk thread in a glass flask, so that the balls were nearly at 

 the same level. Some water was then put in the flask and boiled 

 until all the air was driven out of the flask, which was then 

 corked and allowed to cool. When cold there was a partial 

 vacuum in it, the gauge showing from \ to % of an inch 

 pressure. 



It was now found that when the flame of a lamp was 

 Iirought near to the flask the pith-ball which was nearest the 

 flame was driven away ; and that with a piece of ice the pith 

 was attracted. 



This experiment was repeated under a variety of circumstances, 

 in different flasks and with different balances, the stem being 

 sometimes of glass and sometimes of platinum ; the results, how- 

 ever, vtre the same in all cases, except such variations as I am 

 about to descril e. 



The pith-balls were more sensitive to the heat and coM when 

 the flask \\as cold and the tension within it low, but the effect 

 WES perceptible until the gauge showed about an inch, and even 

 alter that the ice would attract the hall. 



The reason why the repulsion from heat was not apparent at 

 greater ter.sicns was clearly due to the convection currents 

 which the heat generated within the flask. When there was 

 enough vapour, these currents carried the pith with Ihem ; they 

 were, in fact, then sufficient to overcome the forces which other- 

 wise moved Ihe pith.. This was sho\\'n by the fact that when 

 the bar was rot quite level, so that one ball was higher than the 

 other, the currents affected them in different degrees ; also that 

 a different effect could be produced by raising or lowering the 

 position of the flame. 



The ctndition of the pith also perceptibly affected the sensi- 

 tiveness of the balls. AVhen a piece of ice was placed against 

 the side of the glass, the nearest of the pith-balls would be drawn 

 towards the ice, and would eventually stop opposite to it. If 

 allo\^cd to remain in this condition for some time, the vapour 

 vould condense on the ball near the ice, while the other ball 

 would become dry (this would be seen to be the case, and was 

 also shown by the tipping of the balance, that ball against the 

 ice gradually getting lower). It was then found when the ice 

 was removed that the dry ball was insensible to the heat, or 

 nearly so, while that ball which had been opposite to the ice was 

 more than ordinarily sensitive. 



If the flask were dry and the tension of the vapour reduced 

 with the pump until the gauge showed % of an inch, then, 

 although purely steam, the vapour was not in a saturated condi- 

 tion, and the pilh-balls which were dry were no longer sensitive 

 to the lamp, although they would still approach the ice. 



From these two last facts it appears as though a certain 

 amount of moisture on the balls was necessary to render them 

 sensitive to the heat. 



In order that these results might be obtained it was necessary 

 that the vapour should be free from air. If a small quantity of 

 air was present, although not enough to appear in the gauge, the 

 effects rapidly diminished, particularly that of the ice, until the 

 convection currents had it all their own way. This agrees with 

 the fact that the presence of a small quantity of air in steam 

 greatly retards condensation and even evaporation. 



With a dry flask and an air-vacuum, neither the lamp nor the 

 ice piroduced their effects ; the convection currents reigned 

 supreme, even when the gauge was as low as | inch. Under 

 these circumstances the lamp generally attracted the balls and 

 the ice repelled them, i.e. the currents carried them towards the 

 lamp and from the ice ; but by placing the lamp or ice very low 

 the reverse effects could be obtained, which goes to prove that 

 they were the effects of the currents of air. 



These experiments appear to show that evaporation from a 

 suiface is attended with a force tending to drive the surface back, 

 and condensation with a force tending to draw the surface for- 

 ward. These effects admit of explanation, although not quite 

 as simply as may at f^rst sight appear. 



Although there must always be reaction corresponding to the 



visible motion, whenever vapour is driven off from a surface, 

 this visible motion is too small to account for the forces under 

 consideration. But, although it appears to have escaped notice 

 so far, it follows as a direct consequence of the kindk theory of 

 gases that whenever evaporation takes place from the surface of 

 a solid body or a liquid, it must be attended with a reactionary 

 force ecjuivalent to an increase of pressure on the surface, which 

 force is quite independent of the perceptible motion of the 

 vapour. Also condensation nust be attended with a force equiva- 

 lent to a diminution of ihe ga>;eous pressure over the condensing 

 surface, and likewise independent of the vi-ible motion of the 

 vapour. This may be shown to be the case as follows : — 



According to the kinetic theory the molecules which consti- 

 tute the gas are in rapid motion, and the pressure wdiich the gas 

 exerts against the bounding surfaces is due to the successive im- 

 pulses of these molecules, whose course directs them against the 

 surface, from which they rebound with unimpaired velocity. 

 According to this theory, therefore, whenever a molecule of 

 liquid leaves the surface henceforth to become a molecule of gas, 

 it must leavt it with a velocity equal to that whh which the other 

 particles of gas rebound — that is to say, instead of being just de- 

 tached and quietly passing off into the gas, it must he shot off 

 with a velocity greater than that of a cannon-ball. Whatever 

 may be the nature of the forces which give it the velocity, and 

 which consumes the latent heat in doing so, it is certain, from 

 the principle of conservation of momentum, that they must react 

 on the surface with a force equal to that exerted on the molecule, 

 just as in a gun the pressure of the powder on the breech is the 

 tame as on the shot. 



The impulse on the surface, from each molecule which is 

 driven ofi' by evaporation, must therefore be equal to that caused 

 by the rebound of ore of the reflected molecules (supposing all 

 the molecules to be of the same size), that is to say, since the 

 force of rebound will be equal to that of stopping the impulse 

 from a particle driven cff by evaporation will be half the impulse 

 received from the stojping and reflection of a particle of the gas. 

 Thus the effect of evaporation will be to inciease the number of 

 impulses on the surface ; ard, although each of the new impulses 

 will only be half as effective as the ordinary ones, they will add 

 to the pressure. 



In the same way, whenever a molecule of gas ccmes up to a 

 surface and instead of rebounding is caught and retained by the 

 surface, and is thus cordensed into a n olecule of liquid, the im- 

 pulse which it will ihus impart to the surface will only be one- 

 half as great as if it had rebounded. Hence condensation will 

 reduce the magnitude of some of the impulses, ard hence will 

 reduce the pressure on the condensing surface. 



This explanation is then put in a mathematical form, and the 

 paper proceeds. 



Applying these results to steam, wc find that at a temperature 

 of 60" the evaporation of i lb. of water frcm a surface would be 

 sufficient to maintain a force of 65 lbs. for one second. 



It is also important to notice that this force will be propoitional 

 to the square root cf the absolute temperature, and consequently 

 will be approximately constant betw een temperatures of 32° and 

 212°. 



If we take mercury instead of water, we find that the force is 

 only 6 lbs. instead of 65 ; but the latent heat of mercury is only 

 jV that of water, so that the sarce expenditure of heat would 

 maintain nearly three times as great a force. 



It seems, therefore, that in this way we can give a sathifactory 

 explanation of the experiments previously described. When 

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

 perature will rise, and any moisture on it will begin to eva- 

 porate, and to drive the ]^ith from the lamp. The evapo- 

 ration will be greatest on that ball which is nearest to the 

 lamp, therefore this ball will be driven away until the 

 force on the other becomes equal, after which the balls wUl 

 come to rest, unless momentum carries them further. On the 

 other hand, w-hen a piece of ice is brought near the temperature 

 of the pith it will be reduced, and it will condense the vapour 

 and be drawn towards the ice. 



The reason why Mr. Crcokes did not obtain the same results 

 with a less perfect vacuum was because he had then too large a 

 proportion of air or non condcnsirg gas mixed with the vapour, 

 which also was not in a state of saturation. In the expeiiments 

 the condensable vapour was that of mercury, or something ^^■hich 

 required a still higher temperature, and it was necessary that the 

 vacuum should be very perfect for such vapour to be anything 

 like pure and in a saturated condition. As soon, however, as 

 this state of perfection was reached, then the effects were more 



