May (), 1878] 



NATURE 



55 



•tern of temperatures due to a solid sphere of uniform tempera- 

 ture, inmiersed in the gas, cannot of itself give rise to any force 

 tending to move the sphere in one direction rather than in 

 another. Let the sphere be placed within the finite portion of 

 gas which, as we have said, is already in equilibrium. The 

 equilibrium will not be disturbed. We may introduce any num- 

 ber of spheres at different temperatures into the portion of gas, 

 and when the flow of heat has become steady, the whole system 

 will be in equilibrium. 



12. How, then, are we to account for the observed fact that 

 forces act between solid bodies immersed in rarefied gases, and 

 this, apparently as long as inequalities of temperature are 

 maintained ? 



I think we must look for an explanation in the fact dis- 

 covered in the case of liquids by Helmholtz and Piotrowski,^ 

 and for gases by Kundt and Warburg,' that the fluid in contact 

 with the surface of a solid must slide over it with a finite velocity 

 in order to produce a finite tangential stress. 



The theoretical treatment of the boundary conditions between 

 a gas and a solid is difficult, and it becomes more difficult if we 

 consider that the gas close to the surface is probably in an 

 unknown state of condensation. We shall, therefore, accept 

 the results obtained by Kundt and Warburg on their experi- 

 mental evidence. 



They have found that the velocity of sliding of the gas over 

 the slurface due to a given tangential stress varies inversely as 

 the pressure. 



The coefficient of sliding _for air on glass was found to be 



A. = _ centimetres, where / is the pressure in millionths of an 



/ ■ 



atmosphere. Hence at ordinary pressures \ is insensible, but in 

 the vessels exhausted by Mr. Crookes it may be considerable. 



Hence if close to the surface of a solid there is a tangential 

 stress, S, acting on a surface parallel to that of the body, in a 

 -direction h, parallel to that surface, there will also be a sliding 

 of the gas in contact with the solid over its surface in the 



direction h, with a finite velocity = S -. 



13. I have not attempted to enter on the calculation of the 

 effect of this sliding motion, but it is easy to see that if we begin 

 Avith the case in which there is no sliding, the effect of per- 

 mission being given to the gas to slide must be in the first place 

 to diminish the action of all tangential stresses on the surface 

 without affecting the normal stresses ; and in the second place 

 to set up currents sweeping over the surfaces of solid bodies, 

 thus completely destroying the simplicity of our first solution 

 of the problem. 



14. When external forces, such as gravity, act on the gas, 

 and when the thermal phenomena produce differences of density 

 in different parts of the vessel, then the well-known convection 

 currents are set up. These also interfere with the simplicity of 

 the problem and introduce very complicated effects. All that we 

 know is that the rarer the gas and the smaller the vessel, the 

 less is the velocity of the convection currents ; so that in Mr. 

 Crookes's experiments they play a very small part. 



Mathematical Society, April 11. — C. W. Merrifield, 

 F.R.S., vice-president, in the chair,— Mr. Artemas Martin, 

 Pennsylvania, was elected a Member, and Messrs. W. M. Hicks 

 and T. R. Terry were proposed for election. — The Chairman, 

 on the recommendation of the Council, nominated Messrs. 

 Brioschi, Darboux, Gordan, Sophus Lie, and Mannheim for the 

 honour of Foreign Membership. — Prof. H.J. S. Smith, F.R.S., 

 vice-president, read two papers : second notice on the charac- 

 teristics of the modular curves, and a note relating to the theory 

 of the division of the circle, — Mr. Tucker communicated a letter 

 from Prof. Tait, and read an abstract of a paper by Prof. 

 Minchin on the astatic conditions of a body acted on by given 

 forces, and a portion of a paper by Mr, C, Leudesdorf on certain 

 extensions of Frullani's theorem. 



Royal Astronomical Society, April 12. — Lord Lindsay, 

 president, in the chair. — A paper was read by Capt. Abney, 

 R.E., F.R.S., on photography at the least refrangible end of 

 the solar spectrum, and some photographs of spectra of great 

 interest were exhibited to the Fellows. Some discussion ensued, 

 and Dr. De la Rue asked a question respecting colour photo- 

 graphy; Capt. Abney attributed such phenomena to different 

 -degrees of oxidation of the spectrum. The President brought 

 iip Dr. Draper's discoveries, but Capt. Abney declined to speak 



' Wiener Sitzb., xl. (i860), p. 607. 

 2 Pogg. Ann., civ. (1875), p. 337. 



upon that subject. — The Astronomer- Royal remarked upon the 

 proposal to set up a statue of the late M. Leverrier, showing 

 that the gratification and pride which the neighbours of such a 

 great luminary would naturally take in setting up his monument 

 ought not to be snatched from them by the intervention of 

 strangers. It was also pointed out that the charter of the 

 Society does not admit of any subscription in its corporate 

 capacity. — Mr. Christie read a letter from Mr. EUery upon Mars 

 at opposition, 1877. It appeared that the planet was very ill- 

 defined, and not much good could be done with it. — Mr. B. G. 

 Jenkins read a paper on the transit of Mercury, summarising the 

 history of such phenomena, and referring to the spot of light on 

 the disc and the ring round the limb, and their variations corre- 

 sponding with perihelion and aphelion transits, Mr, Chambers 

 suggested that if this paper were published before next transit 

 it would be of great value, whereas, according to the prac- 

 tice of the present editor of the Notices, that would not be 

 done. Prof. Cayley, the editor, made the proper excuses for 

 the lateness of the publication, — The Astronomer-Royal an- 

 nounced his intention to assist competent observers, who wished 

 to observe the transit, by giving them the use of the telescopes 

 which were employed for the transit of Venus, — Mr. Green read 

 a letter from Prof. Schiaparelli on Mars as seen recently a long 

 time after opposition, and showed some ciurious drawings. 



Chemical Society, April i8. — W. Crookes, F.R.S., in the 

 chair. — The following papers were read : — On terpin and ter- 

 pinol, by Dr. Tilden. The author prepared crystallised terpin, 

 C10H20O2 , OHj, by Wigger's process, and obtained the same 

 compound from American and French terpentine, but did not 

 procure any crystalline substance from the ter penes of the 

 orange group. By the action of dilute hydrochloric acid on 

 terpin, an oily body, terpinol, boiling 205°-2i5°, was obtained, 

 having the formula, CjoHigO. By the action of dry hydro- 

 chloric acid on terpinol, a dihydrochloride was prepared. The 

 author believes that in the preparation of terpin by the ordinary 

 process, terpinol is formed at a certain stage of the reaction. 

 By acting on terpin with dilute sulphuric acid, a hydrocarbon, 

 CjqHjs, boiling at 1 76°-! 78°, sp. gr. 0*8526 was obtained ; it is 

 optically inactive, and gives no crystalline deposit with hydro- 

 chloric acid, and no crystalline nitroso compound ; the author 

 proposes to call it terpinylene. — The poisonous principle of 

 Urechites subereda, by J. J. Bowrey. This plant grows wild in 

 Jamaica ; it^ has dark green leaves and large bright yellow 

 flowers; it is locally called "nightshade." It is known to be 

 very poisonous. The author has extracted from the fresh leaves 

 of the plant, by the use of alcohol, water, and a temperature 

 not exceeding 38° C, a white crystalline l^pdy, urechitin, 

 C28H42O8, to the presence of which the plant owes its poisonous 

 properties. It is very soluble in hot alcohol, chloroform, and 

 glacial acetic acid ; almost insoluble in water and dilute spirit. 

 It is intensely bitter, and very poisonous ; it gives, with strong 

 sulphuric acid, a characteristic coloiur reaction. The liquid passing 

 from yellow through red to purple, a trace of nitric acid in- 

 creases the rapidity of the colour-changes. If the leaves are 

 dried at 100°, urechitoxin is obtained, either crystalline or amor- 

 phous. This substance resembles urechitin in its chemical and 

 toxical properties. Both substances are glucosides. — The tem- 

 perature at which some of the alkaloids, &c., sublime as deter- 

 muied by an improved method by A. W. Blyth. The author 

 has determined the melting and subliming points of many 

 active vegetable principles, and classed them as regards their 

 behaviour to heat for practical purposes. He has also devised 

 a new method for determining subliming points : -it consists 

 essentially in placing the substance on a thin cover glass floating 

 on a bath of mercury, and examining a second cover glass 

 placed over the substance, from time to time with a 5 -inch 

 objective, the mercury being gradually heated. 



Entomological Society, April 3. — H. W, Bates, F,L,S., 

 F.Z.S., president, in the chair. — Miss E. A. Ormerod was 

 elected a Member of the Society. — Mr. McLachlan remarked 

 that the opinion expressed by Mr. J. P. M. Weale at the last 

 meeting as to the functional purpose of the cephalic process in 

 Termes triniverius, was corroborative of an observation already 

 recorded in Hagen's " Monographic der Termiten." — Mr. F. 

 Grut exhibited, on behalf of the Rev. T. A. Marshall, a col- 

 lection of insects which that gentleman had made in the Wind- 

 ward Islands. — Mr. F. Smith exhibited a series of specimens 

 of a species of "harvesting ant" sent to Mr. Darwin from 

 Florida by Mrs. M. Treat. Three series showed a gradation 

 from large soldiers and small workers, all having acutely dentate 



