88 



NATURE 



{Nov. 28, 1878 



its appendages, and the other abdominal and thoi-acic viscera, 

 — Dr. Creighton publishes his- observations on the supra-renal 

 bodies based on microscopical investigations of these organs when 

 adult and during development, and shows how they present many 

 features of analogy to the ovaries. — Prof. Humphry gives his 

 reasons for dissent from Dr. Ogston's views on the important 

 share taken by articular cartilage in the growth of bone, as 

 expressed in the July number of the Journal. — Prof. Turner 

 describes the placentation of the hog-deer (Cervus porcinus). — 

 Dr. Urban Pritchard supplements his previous accounts of the 

 development of the organ of Oorti in the internal ear. — Dr. T. 

 B, Henderson, of Glasgow, describes the physiological effects of 

 the inhalation of phosphuretted hydrogen. 



Journal de Physique, October. — In this Enumber Prof. Dufet 

 studies the variation of the indices of refraction in mixtures of 

 isomorphous salts, arriving at the experimental law that the 

 differences between the indices of a mixture of two such salts 

 and those of the component salts are in inverse ratio of the 

 numbers of equivalents of the two salts forming the mixture ; 

 in other terms the curve which has for ordinates the indices and 

 for abscissae the equivalents, is a straight line. This law is 

 regarded as a consequence of Gladstone's, on the constancy of 

 specific refractive energy in mixtures. — M. Terquem describes an 

 improved way of realising Plateau's liquid laminar systems, 

 giving larger systems with less liquid. Instead of using pieces 

 all rigid, he uses a combination of rigid pieces with flexible 

 threads (silk), ,?."•., two horizontal rods joined together at the 

 ends with such threads or two rings joined with threads. Many 

 instructive effects are thus had. The liquid used is a solution 

 of soap and sugar, prepared in a special way. — M. Bouty con- 

 tributes a mathematical paper on the number of elements neces- 

 sary for determining the exterior effect of an optical system, and 

 M. Bichat gives a new method of measuring the velocity of 

 sound. 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, November 21. — "On Repulsion resulting 

 from Radiation." Part VI., by William Crookes, F.R.S., 

 V.P.C.S. 



In this part, with which the research closes, the author first 

 examines the action of thin mica screens fixed on the fly of an 

 ordinary radiometer, in modifying, the movements. It is found 

 that when a disk of thin clear mica is attached i millim. in 

 front of the blacked side of the vanes of an ordinary radio- 

 meter, the fly moves negatively, the black side approaching 

 instead of retreating from the light. When a thin mica disk is 

 fixed on each side of the vanes of a radiometer, the result is an 

 almost total loss of sensitiveness. 



In order to examine the action of screens still further an 

 instrument is described having the screens movable, and working 

 on a pivot independent of the one carrying the fly, so that the 

 screens can move freely and come close either to the black or to 

 the white surfaces of the disks. By gentle tapping the screens 

 can be brought within 2 millims. of the black surface.-. A 

 candle is now brought near, shaded so that the light has to pass 

 through one of the clear disks and fall on the black surface. 

 The black side immediately retreats, the clear disk remaining 

 stationary for a moment, and then approaching the light. If 

 the candle is allowed to shine on the plain side of the black 

 disk, no immediate movement takes place. Very soon, how- 

 ever, both disks move in the same direction away from the 

 candle, the speed of the clear disk gradually increasing over that 

 of the blacked disk. 



Instead of allowing the clear screens to freely move on a 

 pivot, an instrument was made in which the screens could be 

 fixed beforehand in any desired position in respect to the blacked 

 disk. It was then found that with the screens close to the blacked 

 sides of the vanes the fly rotates very slowly in the negative 

 direction, stopping altogether when the candle is moved five or 

 six inches off. \Vith the screens l millim. from the black 

 surface the direction is negative, and the speed at its maximum. 

 When the screens and disks are 7 millims. apart a position of 

 neutrality is attained, no movement taking place. When the 

 distance is further increased, positive rotation commences, 

 which gets stronger as the screens approach the bright sides of 

 the disks, where the positive rotation is at its maximum. The 

 author adduces reasons for considering that the negative rota- 



tions here observed are caused by the warming up of the black 

 surface by radiation falling direct on it, through the clear mica 

 screen, and the deflection back\\ards of the lines of molecular 

 pressure thereby generated. 



The action of these radiometers being complicated, owing to 

 the surfaces of the vanes being different in absorptive power, 

 another instrument was made in which the vanes were of 

 polished aluminium, perfectly flat and symmetrical with the bulb. 

 The screens were of clear mica movable in respect to the vanes, 

 and at right angles to their surface. When exposed to the light 

 of a candle it was found that with the screens brought up close 

 to the disks, the rotation was as if the unscreened side were 

 repelled ; at an intermediate position there was neutrality. 

 Explanations are given of these movements, but without the 

 illustrative cuts they would be unintelligible. 



Experiments on radiometers having movable screens inter- 

 posed between the vanes and the bulb are next given, and these 

 are followed by a long series of experiments on the influence of 

 movable screens on radiometers with cup-shaped metallic vanes, 

 the screens being varied in shape and position in respect to the 

 plane of rotation, as well as in respect to the distance from the 

 vanes. 



A similar series is given with metallic cylinders as vanes, and 

 from the behaviour of the latter kind of radiometer an expla- 

 nation is given of the various movements previously obtained. It 

 is found that when the screen touches the convex surface of the 

 vanes the rotation under the influence of light is always positive. 

 It commences at a low exhaustion, increases in speed till the 

 rarefaction is so high that an ordinary radiometer would begin 

 to lose sensitiveness, and afterwards remains at about the same 

 speed up to the highest rarefaction yet obtained. At any 

 rarefaction after 87 M (millionths of an atmosphere) there is a 

 neutral position for the screen. When it is on the concave 

 side of this neutral position the direction of rotation is positive, 

 and when on the convex side of the neutral position it is nega- 

 tive ; the speed of rotation is greater as the vanes are further 

 removed from this neutral position on either side. The position 

 of this neutral point varies with the degree of exhaustion ; thus 

 at 12 M the screens must be 3 millims. from the convex 

 side ; at 'iS M they must be 13 millims. from the convex side. 

 The higher the exhaustion the greater the distance which must 

 separate the convex side of the hemi-cylinders and the screens. 



The author gives explanations of these phenomena based on 

 the following already ascertained facts : — When thin aluminium 

 vanes are exposed to light the metal rises in temperature and 

 becomes equally warm throughout, and a layer of molecular 

 pressure is generated on its surface. The thickness of this layer 

 of fwessure, or the length of the lines of force of repulsion 

 varies with the degree of exhaustion, being longer as the ex- 

 haustion increases. The lines of force appear to radiate from 

 the metal in a direction normal to its surface. The force of 

 repulsion is also greater the closer the repelled body is to the 

 generating or driving surface, and the force diminishes rapidly 

 as the distance increases, according to a law which does not 

 appear to be that of "inverse squares." Diagrams are given 

 illustrating the author's explanation based on the above data. 



An apparatus is next described not differing in principle from 

 the last, but having, in addition to the aluminium hemi-cylinder 

 and movable mica screen, a small rotating fly made of clear mica, 

 mounted in such a way that it could be fixed by means of an 

 exterior magnet in any desired position inside the bulb. The 

 screen was also capable of adjustment by means of another 

 magnet ; the aluminium hemi-cylinder in jthis apparatus being 

 fixed immovably. The adjustible indicator being very small in 

 diameter in comparison to the other parts of the apparatus, -and, 

 being easily placed in any part of the bulb, was expected to aftord 

 information as to the intensity and direction of the lines of pres- 

 sure when a candle v as brought near the bulb. Experiments 

 have been tried, a, with the screen in different positions in 

 respect to the hemi-cylinder ; b, with the indicator in different 

 parts of the bulb ; c, "with the candle at different distances from 

 the hemi-cylinder on one side or the other ; d, with the degn"ee 

 of exhaustion varying between wide limits. It would be im- 

 possible to give an intelligible abstract of the results obtained 

 with this apparatus without numerous diagrams It may, how- 

 ever, be briefly stated that they entirely corroborate the theories 

 formed from a study of the behaviour of the instruments 

 previously described. 



The next part of the paper treats of the action of heat em- 

 ployed ini^ide the radiometer. In a previous paper the author 



