SCIENCE. 



219 



nouncement startles us by its boldness, and encounters 

 the opposition of our conservatism. I shall have the 

 pleasure of reproducing a part of Mr. Crooke's experi- 

 ments before >ou with tubes made after his models, and 

 recently imported ; and at the close of the exhibition I 

 hope you will be able to draw your own conclusions. 



Let us first prepare the way from our study of the sub- 

 ject by a clear presentation to our minds of the exact 

 differences between the solid, the liquid, and the gaseous 

 states. We shall then be able the better to judge whether 

 this new claimant is sufficiently differentiated from ordin- 

 ary gases to warrant its being set aside by itself. 



I. A solid is composed of distinct molecules separated 

 from one another by spaces which are large compared 

 with the diameter of molecules themselves. These mole- 

 cules, which are the indivisible units so far as all physical 

 changes are concerned, are held rigidly in a fixed relation 

 to one another by cohesion, a force which acts only across 

 these invisible inter-molecular spaces, and the intensity of 

 which depends upon the chemical constitution of the 

 body. The force of cohesion is counterbalanced by the 

 motion of the individual molecules, the energy of this 

 motion constituting the heat of the body. As the tem- 

 perature rises, the amplitude of vibration increases, and 

 the force of cohesion is diminished by an increase of dis- 

 tance between the molecules. When the temperature falls 

 the reverse process ta^es place. 



Molecules of matter, in the solid state, retain fixity of 

 position about their centres of oscillation ; and any at- 

 tempt to change the relation of these centres, by distort- 

 ing the body, calls into action resistances opposing the 

 change of form. 



II. When the temperature rises so high that the mole- 

 cules lose their fixity of position, and are released from 

 the rigid thraldom of cohesion, so that they can appar- 

 ently roll round one another without changing the dis- 

 tances of their centres, then the solid becomes liquid. As 

 the force of cohesion differs with each kind of matter, 

 so the temperatures of liquefaction also differ. In liq- 

 uids the cohesion is much diminished but not entirely 

 overcome by the energy of molecular oscillations, which 

 we call heat. A liquid is characterized by taking the 

 form of the containiug vessel, by its surface becoming 

 horizontal, and by its equal transmission of applied pres- 

 sure in all directions. The state of liquidity is due to in- 

 termolecular motions of much greater amplitude than 

 those peculiar to the solid state. 



III. The temperature still rising, the molecules are 

 finally released entirely from the force of cohesion, and 

 move about in every conceivable direction with enormous 

 and constantly varying velocities. The mean distance 

 between them is sufficiently great to render cohesion in- 

 operative ; but they come into constant collisions with 

 one another, now colliding, and now rebounding, chang- 

 ing their direction of motion aDd perhaps their velocity 

 with every impact. On account of this rapid molecular 

 motion and the absence of cohesion in gases, they tend 

 to expand indefinitely ; and they transmit pressure by 

 multitudinous collisions among the molecules, and exert 

 pressure on the walls of the containing vessel by innum- 

 erable impacts against its interior. Place a small, closed 

 balloon, partly filled with air, under the receiver of an 

 airpump and exhaust. The ballcon distends more and 

 more as the exhaustion proceeds, because of this mole- 

 cular bombardment against its inner surface, the ex- 

 ternal coun eracting pressure being withdrawn.' In much 

 the same way an impenetrable sheet of metal might be kept 

 suspended horizontally in the air by bullets fired vertically 

 against its under surface. The dynamical principles con- 

 cerned in the two cases are identical. When a gas is 

 heated, the velocity of the individual molecules increases 

 and they exert greater pressure on the walls of the con- 

 taining vessel by their impact, or by expansion secure a 

 longer free path for themselves between successive col- 

 lisions with one another. This mean free path, or mean 



length of path, is known to be about tsoVutj of an inch 

 at the average or normal pressure of the air, while the 

 mean distance between the molecules is about one 

 seven-millionth of an inch, according to the late Prof. 

 Clerk Maxwell. Accordingly a cubic inch of air at nor- 

 mal density must contain the cube of 7,000,000 or 343 

 quintillions of molecules. Each of these molecules, more- 

 over has, perhaps, millions of encounters every second, 

 the number varying with the kind of gas considered. The 

 state of gaseity is therefore pre-eminently characterized by 

 innumerable molecular collis:ons, from which proceed all 

 the properties constituting the ordinary gaseous state of 

 matter ; and this state continues so long as the motions 

 of the molecules are in every possible direction, and the 

 collisions almost infinite in number. Under such con- 

 ditions the mean length of path of the molecules, or the 

 distance traversed between two successive collisions, is 

 extremely small compared with the dimensions of the 

 containing vessel, only TsoVinj of an inch. 



Such are the distinctions existing between the solid, 

 the liquid, and the gaseous states, distinctions depend- 

 ing entirely upon molecular aggregation. In the snow 

 or frost crystal, for instance, the molecules of water are 

 grouped together in a definite structural relation ; the ap- 

 plication of heat destroys the structure, the molecules be- 

 coming more nearly individualized by a certain increase 

 of independence or freedom of motion, and the solid be- 

 comes a liquid ; a still further rise of temperature in- 

 ci eases this molecular independence to such an extent 

 that their only physical relationship depends upon innum- 

 erable collisions with one another. If now, by any means, 

 molecular independence can be rendered so nearly com- 

 plete that the hits may be disregarded in comparison 

 with the misses, then the free path of the molecule be- 

 tween the hits may become comparable with the dimen- 

 sions of the containing vessel, the properties which con- 

 stitute gaseity are reduced to a minimum, and matter is 

 exalted to an ultra-gaseous state. The residual gas is 

 then in that peculiar state when it has ceased to 

 have the power of adjusting its own pressure ; and con- 

 sequently the phenomenon of diverting the molecules by 

 suitable means into any paths at desire will be possible. 

 Such is the ultimate result of gaseous expansion. As 

 the rarefaction proceeds the number of molecules in a 

 given space becomes smaller and smaller, and the mean 

 length of path greater and greater, the molecular collis- 

 ions diminishing as the free path increases. 



At an exhausiion of a millionth of an atmosphere the 

 number of molecules in any given space is reduced to 

 one millionth of the number at the ordinary pressure ; 

 each molecule has a million times more room to move 

 about in, and its mean length of path becomes about four 

 inches (1,000,000 times tttuVuu in.) If now the rarefac- 

 tion be carried to another millionth, the space appro- 

 priated to each molecule is then increased a million mil- 

 lion times, but the distance between the molecules be- 

 comes only one seven-hundredth of an inch, and a cubic 

 inch contain seven then not less than 340 millions of mole- 

 cules, according to Prof. Clerk Maxwell. This rarefac- 

 tion is about 50,000 times further than the best mercurial 

 air-pump can attain. A gas transmits pressure instan- 

 taneously in all directions by innumerable molecular 

 collisions ; an ultra qas, or "radiant matter," has lost the 

 power of adjusting its pressure because of the infre- 

 quency of collisions among the molecules. 



If then a bulb about four inches in diameter is ex- 

 hausted to a millionth of an atmosphere, the residual gas 

 in it will have lost the power ot rapid adjustment of 

 pressure to equality at every point. Hence any extran- 

 eous force, like heat or electricity, may "infuse order into 

 the apparently disorderly jostling of the molecules in 

 every direction by coercing them into a methodical recti- 

 linear movement. ***** And according to the extent 

 to which this onward movement has replaced the irreg- 

 ular motions, which constitute the very essence of the 



