September 1, 1892.] 



KNOWLEDGE 



177 



tuate the fact that for all essential purposes the slii,'ht 

 differences between them are simply of degree and not of 

 kind. 



We have adopted provisionally, and merely for the sake 

 of simplicity, the view that inter-molecular motions (the 

 relative motions of molecules as wholes) take place in 

 perfectly straight lines. This is certainly a near ap- 

 proximation to the truth. It is therefore reasonable 

 to expect that in gases of ordinary density — in the open air 

 for instance — eflects identical with those described under 

 " Radiometry " in the July issue of Knowledge should be 

 obtainable ; all that is necessary being that the hot and 

 cold surfaces should be brought sufficiently closely into 

 proximity. This has been accomplished in several ways : 

 notably, by means of an " otheoscope " specially constructed 

 with this object by Prof. Crookes. Another example, 

 familiar to everyone, of molecular pressure exhibited in gas 

 not much removed from the normal density is furnished 

 by the so-called calorific parados or spheroidal state. 

 When a globule of water is placed on a red-hot stove-plate, 

 evaporation gives rise to such a shower of radiant molecules, 

 raining down on the hot metal from the lower surface of 

 the liquid spheroid, that a mechanical reaction is produced 

 equal to sustaining the weight of the drop, so that this 

 rolls on, without touching, the heated surface. Everyone 

 must have noticed the rapid tremulous agitation of a drop 

 "of water undergoing slow evaporation in this way ; and it 

 may not be out of place to suggest, for illustrative purposes, 

 that these oscillations exemplify very crudely what is 

 meant by intra-molecular ^ibration ; only, of course, just 

 as a molecule is very much smaller, so its vibrations are 

 very much more rapid than anything within the cognizance 

 of our senses. 



Now to return to the consideration of gases so condi- 

 tioned that the peculiarities of their inter-molecular motions 

 may be most directly and readily shown. The description 

 of only a few of the experiments by which Prof. Crookes 

 has so thoroughly explored these matters, will give exten- 

 sion and confirmation to such explanations as have already 

 been given. One of the first questions that presents itself 

 is, Does speinlar reflection take place at the surface of a 

 body from which a molecule rebounds, and is the angle of 

 rebounding equal to the angle of incidence '.' The 

 answer to this question is in the negative ; for if specular 

 reflection occurred, the only effect that could be produced 

 by the impact of molecules would be a pressure norimd to 

 the surface struck. The oblique component, if any, of the 

 molecular pressure, acting in the line of incidence, would be 

 exactly neutralized by the oblique reaction in the line of 

 reflection. But it is weU known that the obUque com- 

 ponent of molecular pressure is not compensated in this 

 way. Fig. 1 shows a form of radiometer by which this 

 fact is very prettily demonstrated. The 

 skewed vanes are, as usual, coated with 

 lampblack, but do not rotate. They are 

 . permanently fixed to the upright in the 

 position shown. Above them is a movable 

 disc of thin, smooth mica, supported in 

 the centre upon a needle point. The 

 upper surface of the disc is painted in 

 sectors with pigments representing the 

 different colours of the spectrum duly 

 proportioned ; and it is made to rotate 

 with such rapidity by the oblique streams 

 of molecules fi'om the skewed lamp- 

 blacked vanes, that the colours blend 

 with one another to a neutral grey. Al- 

 though it is true that some highly polished 

 sm-faces are moved by the oblique com- 



Fio. 1. 



ponent of molecular pressure in a way inconsistent with 

 simple reflection, there is of course considerable loss of 

 power in such an application of the molecular pressure 

 to the production of molar motion. Thus with the 

 ordinary form of radiometer one is not surprised to 

 learn that much more energetic eflects are obtained when 

 a narrow hoop of metal, transversely stepped or corrugated 

 and painted with lampblack, is placed, for a reacting surface, 

 horizontally within the glass globe. Under this arrange- 

 ment the energy of the projected molecules is more 

 perfectly absorbed at the moment of impact; and the 

 "cooled," i.e. retarded, molecules may even be returned 

 directly back upon the vane or fly. Indeed, a single mole- 

 cule may thus be bandied very many times in succession 

 between the vane and the reacting surface ; and Prof. 

 Crookes has pointed out that with a sufficient degree of 

 rarefaction differences of molecular pressure may exist 

 between different parts of the apparatus for twenty minutes 

 or longer. 



It has been asserted that the molecular wind from a 

 heated surface blows verynearly normally to the surface ; but 

 the fact that radiant matter is also thrown ofl" in oblique 

 directions has been shown by the following experiment. 

 A single " vane " resembling the half of a cone of perhaps 

 one inch in height was made (hollow for the sake of 

 lightness) out of thin sheet mica, and it was fixed on the 

 end of an arm, centrally pivoted, in the same way as a 

 magnetic compass needle. The opposite extremity of the 

 arm carried a small metal counterpoise ; and the whole 

 was mounted in a radiometer bulb, exhausted of course, 

 and having within it, below the pivoted arm, a large 

 horizontal disc of lampblacked material. The semicircular 

 base of the mica vane was horizontal, and therefore 

 parallel with the blackened or radiant surface. The 

 triangular (flat) surface of the vane was normal to 

 the blackened disc, and its apex was uppermost. Hence 

 molecules thrown off vertically from the blackened surface 

 would simply exert an upward pressure on the semi- 

 circular base of the vane. They could not act in any way 

 upon either the straight or the curved side thereof. 

 Molecules projected obliquely might, on the other hand, 

 impinge on the vertical plane side, or, but to a less ejctent, 

 on the sloping curved side of the vane. For the same 

 degree of obliquity of the molecular showers, the angle of 

 incidence would be great on the vertical side and small on 

 the sloping side of the vane. Hence it is clear, upon the 

 whole, that oblique molecular currents would be deflected 

 to the greatest extent by the vertical plane surface ; and 

 that their existence involves the manifestation at this 

 surface of an excess of molecular pressure. As a matter 

 of fact, the pressure on the plain side of the vane is enough 

 to set the arm in rotation. 



Radiometers have been made in which one of the 

 horizontal arms to which the vanes are attached is a 

 magnetic needle. Of course, the rotation of the vanes in 

 such an instrument can be stopped by simply bringing a 

 powerful bar magnet into the neighbourhood of the bulb. 

 If this is done while the instrument floats, like a hydro- 

 meter, in a glass jar filled with water, the showers of 

 molecules from the stationary vanes wiU set the glass 

 envelope itself into rotation. A paper index, of say one 

 foot in length, may be attached to the upper surface of the 

 glass globe and will make this revolution more evident. 

 Such an arrangement, in which the motion produced by 

 light is not only visible but tangible, makes an exceedingly 

 attractive scientific toy. Prof. Crookes suggests also that 

 the rotation of the magnetic needle within such an instru- 

 ment as this might be made to induce electric currents in 

 wires placed outside the bulb, thereby operating self- 



