Dec. 12, 1878] 



NATURE 



139 



The plate, however, being thick, bears the hammering without 

 growing hot enough to lose its power of phosphorescing. 



Mechanical Action of Projected Molecules 



When the coil was first turned on, the thin film was driven 

 back at the moment of becoming phosphore-cent, showing that 

 an actual material blow had Ijeen given by the molecules. 

 Experiments are next described in which this mechanical action 

 is rendered more evident. A small rotating fly, capable of 

 being moved about in any part of an exhausted bulb, is used as 

 an indicator, and by appropriate means the molecular shadow of 

 an aluminium plate is projected along the bulb. Whether 

 entirely in, or entirely out of the shadow, the indicator scarcely 

 moves, but when immersed so that one-half is exposed to 

 molecular impact, the fly rotates with extreme velocity. 



Magnetic Deflection of Lines of Molecular Force 



With this apparatus another phenomenon was investigated. 

 It is found that the stream of molecules, whose impact on the 

 glass occasions evolution of light, is verj- sensitive to magnetic 

 influence, and by bringing one pole of an electro-magnet — or even 

 of a small permanent magnet — ^near, the shadow can be twisted to 

 the right or to the left. 



When the little indicator was placed entirely within the 

 molecular shadow, no movement was produced. As soon, how- 

 ever, as an adjacent electro-magnet was excited, the shadow was 

 twisted half oflF the indicator, which immediately rotated with 

 great speed. 



The Trajectory~x}f Molecules 



The amount of deflection of the stream of molecules forming 

 a shadow is in proportion to the magnetic power employed. 



The trajectory of the molecules forming the shadow is cmrved ; 

 when under magnetic influence the action of the magnet is to 

 twist the trajectory of the molecules round in a direction at an 

 angle to their free path, and to a greater extent, as they are 

 nearer the magnet : the direction of tw ist being that of the 

 rfectric current pa=sing round the electro-magnet. 



Laws of Magnetic Deflection 



An apparatus was constructed so that the deflection of a spot 

 of light was used instead of that of a shadow, a horse-shoe magnet 

 being placed underneath the negative pole to deflect the tra- 

 jectory. The action of the north pole being to give the line of 

 molecules a spiral twist one way, and that of the south pole being 

 to twist it the other way, the two poles side by side compel the 

 line to move in a straight line up or down along a plane at right 

 angles to the plane of the magnet and a line joining its poles. 



The ray of molecules does not appear to obey Ampere's law, 

 as it would were it a perfectly flexible conductor, joining the 

 n^iative and the positive pole. The molecules are projected 

 from the negative, but the position of the positive pole, whether 

 in front, at the side, or even behind the n^ative pole, 

 has no influence on their subsequent behaviour, either in pro- 

 I ducing phosphorescence, or mechanical effects, or in thex 

 magnetic deflection. The magnet gives their line of path a spiral 

 twist greater or less according to its power, but diminishing as 

 the molecules get further oflf. 



Numerous experiments were tried in this apparatus with 

 erent gases, and with the magnet in and out of position. 



Working with exhausted air it was found that the spot of 



en phosphorescence on the screen is visible at an exhaustion 



I02'6 M, when the mean free path of the molecules, 



isured by the thickness of the dark space round the negative 



'.i, is only 12 mm. Hence, it follows that a ntmib«r of 

 -lolecules sufficient to excite green phosphorescence on the 

 ■creen are projected the whole distance from the pole to the 

 ~;reen, or 102 mm., without being stopped by collisions. 



Alteration of Molecular Velocity 



If we suppose the magnet to be permanently in position, and 

 s to exert a uniform do^^uward pull on the molecules, we 

 iPerceive that their trajectory is much curved at low exhaustions, 

 (and gets flatter as the exhaustion increases. A flatter trajectory 

 icorresponds to a higher velocity. This may arise from one of 

 two conditions ; either the initial impulse given by the negative 

 jpole is stronger, or the resisting medium is rarer. The latter is 

 brobably the true one. The molecules which produce the green 

 Iphosphorescence must be looked upon as in a state diflfering from 



.se arrested by frequent collisions. The latter impede the 



velocity of the free molecules and allow longer time for mag- 

 netism to act on them ; for, although the deflecting force of 

 magnetism might be expected to increase with the velocity of the 

 molecules. Prof. Stokes has pointed out that it would have to 

 increase as the square of the velocity, in order that the deflec- 

 tion should be as great at high as at low velocities. 



Comparing the free molecules to cannon-balls, the magnetic 

 pull to the earth's gravitation, and the electrical excitation of the 

 negative pole to the explosion of the powder in the gun, the 

 trajectory will be flat when no gravitation acts,*and curved when 

 under the influence of gravitation. It is also much cur\'ed when 

 the ball passes through a dense resisting medium, it is less 

 cur\-ed when the resisting medium gets rarer ; and, as already 

 shown, intensifying the induction spark, equivalent to increasing 

 the charge of powder, gives greater initial velocity, and, there- 

 fore, flattens the trajectory. The parallelism is still closer if we 

 compare the evolution of light seen when the shot strikes the 

 target, with the phosphorescence on the glass screen from mole- 

 cular impacts. 



Focus of Heat of Molecular Imfact 



The author finally describes an apparatus in which he show s 

 that great heat is evolved when the concentrated focus of rays 

 from a nearly hemispherical aluminium cup is deflected side- 

 ways to the walls of the glass tube by a magnet. By using a 

 somewhat larger hemisphere and allowing the negative focus to 

 fall on a strip of platinum foil, the heat rises to the melting 

 point of platinum. 



. An DUra-gaseojis State of Matter 



The paper concludes with some theoretical speculations on the 

 state in which the matter exists in these highly exhausted vessels. 

 The modem idea of the gaseous state is based upon the supposi- 

 tion that a given space contains millions of millions of molecules 

 in rapid movement in all directions, each having millioris of en- 

 counters in a second. In such a case the length of the mean 

 free path of the molecules is exceedingly small as compared with 

 the dimensions of the vessel, and the properties which constitute 

 the ordinary gaseous state of matter, which depend upon con- 

 stant collisions, are observed. But by great rarefaction the free 

 path is made so long that the hits in a given time may be disre- 

 garded in comparison to the misses, in which case the average 

 molecule is allowed to obey its own motions or laws without 

 interference ; and if the mean free path is comparable to the 

 dimensions of the vessel, the properties which constitute gaseity 

 are reduced to a minimum, and the matter becomes exalted to 

 an ultra-gaseous state, in which the verj' decided but hitherto 

 masked properties now under investigation come into play. 



Rays of Molecular Light 



In speaking of a ray of molecular light, the author has been 

 guided more by a desire for conciseness of expression than by a 

 wish to advance a novel theorj-. But he believes that the com- 

 parison, under these special circumstances, is strictly correct, and 

 that he is as well entitled to speak of a ray of molecular or 

 emissive light when its presence is detected only by the light 

 evolved when it falls on a suitable screen, as he is to speak of a 

 sunbeam in a darkened room as a ray of vibratory or ordinary 

 light when its presence is to be seen only by interposing an 

 opaque body in its path. In each case the invisible line of force 

 is spoken of as a ray of light, and if custom has sanctioned this 

 as applied to the undulatory theory, it cannot be wrong to apply 

 the expression to emissive light. The term emissive light must, 

 however, be restricted to the rays between the negative pole and 

 the luminous screen : the light by which the eye then sees the 

 screen is, of course, undulatory. 



The phenomena in these exhausted lubes reveal to physical 

 science a new world — a world where matter exists in a fourth 

 state, where the corpuscular theory of light holds good, and 

 where light does not always move in a straight line ; but wher^ 

 we can never enter, and in which we must be content to observe 

 and experiment from the outside. 



Chemical Society, December 5. — Dr. Gladstone, president, 

 in the chair. — Prof. Tidy read a lengiii/ and important paper on 

 the processes for determining the organic piurity of potable waters. 

 The conclusions at w hich the author arrives, after many experi- 

 ments and a careful examination of the comparative analyses of 

 over 1,600 waters, may be briefly summed up as follows : The 

 ammonia process furnishes results which are marked by singular 

 inconstancy, and are not delicate enough to allow the recognition 



