February 22. 1894J 



NA TURE 



40: 



of the liquids e^peritnented on were bad conductors of heat, 

 and hence required considerable time for the temperature to 

 become uniform. Differences in temperature could readily be 

 detected by observing if the speed of descent of a small sphere 

 varied at different parts of its path. The author suggested that 

 this fact might be used to determine the thermal ccinductivity 

 of liquids heated at the lop by Forbes' method. The falling 

 sphere would form a thermometer of almost infinitesimd 

 thermal capacity. For most oils, spheres of water coloured 

 with eosin could be employed to determine the viscosity. A 

 water-drop of l mm. radius was found to fall one inch per hour 

 in castor oil at 8° C. To determine the variation of viscosity 

 with temperature a special apparatus was used, with which 

 observations could be made in rapid succession by simply invert- 

 ing the tube containing the liquid and the falling sphere. In 

 Mr. Trouton's viscosity experiments, which were somewhat 

 analogous to those described, surface tension complicated the 

 results considerably ; the author's aim had been to eliminate 

 such disturbing influences. Prof. Everett, in a written com- 

 munication, suggested that the motion of the liquid spheres be 

 checked by using beads of quartz or glass. Lord Rayleigh 

 pointed out, in a letter, that the formula employed related to a 

 solid sphere, and thought it not legitimate to use it for liquid 

 spheres, for the tangential forces at the surface would set the 

 interior liquid in relative motion, and modify the resistance 

 experienced. He also thought the existence of a finite 

 coefficient of sliding friction between two fluids an impossi- 

 bility. Mr. Watson said temperatures might be kept constant 

 for days together by Ramsay and Young's vapour jacket. Dr. 

 Sumpner thought the surface tension of such small spheres of 

 mercury was so very large that they would act practically like 

 solids. The want of solidity might be of importance when the 

 two liquids were very nearly alike in density and other pro- 

 perties. Mr. Blakesley said that at high velocities the falling 

 sphere might get a palpitating motion, in addition to the 

 gradual descent, and this might introduce errors. Prof. Perry 

 considered that the experiments on the velocity of a small 

 sphere, and those of the two parts in which it was divided, 

 which showed that V- = v{- -}- v.^ proved the simple formula 

 used to be correct. Mr. Boys inquired if any tests had been 

 made on the constancy of dimensions of the spheres 

 used. . He would expect that in the case of water and 

 oil, for example, that mutual contamination would take place. 

 Speaking of the indirect method of determining the masses 

 of suiall spheres, he thought direct weighings might be made, 

 for, as the President and Prof. Poynting had shown, the 

 balance might be immensely improved. Prof. S P. Thompson 

 suggested that small globules of aluminium or slag might be 

 used. Dr. C. V. Burton thought Lord Rayleigh's criticism 

 important, and that large corrections might be necessary. 

 He failed to see how the large surface tension mentioned 

 by Ur. Sumpner could prevent internal circulation. Mr. 

 Trotter said Lord Rayleigh's point might be tested by using a 

 sphere of oiled wax. Mr. Boys mentioned that Lord Rayleigh 

 had shown in the case of soap rings that variation of surface 

 tension due to stretching or compression produced stability. 

 The same phenomena would probably retard internal circula- 

 tion. The President said there was little doubt that internal 

 circulation, as mentioned by Lord Rayleigh, would modify the 

 velocity. In his reply, Mr. Jones said he could not imagine how 

 in pure liquids internal motion in the falling spheres could be 

 set up. In answer to Mr. Boys, he had found slight changes in 

 the masses of the water spheres after being used many times, 

 but this was a question of days. During an ordinary series of 

 observation the dissipation was too small to be observed. After 

 the meeting had been adjourned, Mr. Boys and Dr. Burton 

 considered the question of internal circulation, and the latter 

 pointed out that with perfectly liquid spheres there would be 

 infinite slip, and the coefficient of sliding friction /3 would be 

 zero. The velocity of descent would therefore be \ times that 

 given by the first equation. 



Geological Society, February 7.— W. H. Hudleston, 

 F.R.S., President, in the chair. — Mr. C. J. Alford, in expla- 

 nation of specimens of auriferous rocks from Mashonaland 

 exhibited by him, stated that several of them were vein-quartz 

 occurring as segregations in the slates, generally forming veins 

 between the cleavage-planes. Another specimen was a mass of 

 chromate of lead, with pyromorphite and other lead minerals, 

 occurring in masses in decomposed and dislocated talcose slate 



NO. 1269. VOL 49I 



in the Penhalonga Mine near Umtali, and probably resultin.; 

 from the altera ion of masses of galena by weathering, as a 

 broken vein of galena was found in close proximity. This cro- 

 coisite was supposed to be a somewhat rare mineral, but he had 

 found it and also the native red oxide, minium, in several places 

 in South Africa. The most interesting specimen, was, how- 

 ever, a mas^ of diorite showing visible gold througl'out the 

 rock, an assay of which gave upwards of 130 ounces of gold per 

 ton. From information obtained from the prospector who made 

 the discovery, he gathered ihat the deposit was a dyke of diorite 

 running for a consideiable distance, about 8 feet in width, 

 flanked on one side by granite and on the other by slates. 

 There were extensive ancient workings extending to a depth of 

 about 60 feet, and the prospecting shafts had not gone much 

 below that depth, so not much information was obtainable at 

 present. The diorite showed a development of epidote, but 

 little or no quartz ; and the gold appeared to enter in an extra- 

 ordinary manner int;o all of the composing minerals. Mr. Alford 

 hoped, after his next visit to Mashonaland, to be in a position 

 to lay before the Society more definite information regarding 

 these interesting rocks. — The following communications were 

 read : — On some cases of the conversion of compact greenstones 

 into schists, by Prof. T. G. Bonney, F. R.S. By the path lead- 

 ing from the Bernina Hospice to the Grtim Alp (Engadine)' 

 some masses of compact green schist are seen, intercalated in a 

 rather crushed gneiss. They prove to be intrusive dykes modi- 

 fied by pressure. Microscopic examination of specimens from 

 these revealed no trace of any definite structure indicating an 

 igneous rock ; a slice, cut from one of the masses within an 

 inch or so of a junction, showed it to be a foliated mas of 

 minute chlorite or hydrous biotite, with granules of epidote (or 

 possibly some sphene) and of a water-clear mineral, perhaps a 

 secondary felspar. An actual junction showed a less distinct 

 foiiition and some approach to a streaky structure. A slide 

 from the middle of another dyke (about 18 inches thick ) exhibited 

 a more coarsely foliated structure and minerals generally similar 

 to the last, except that it may contain a little actinolite and 

 granules of haematite {?), and the clear mineral, in some cases, 

 seemed to be quartz. The structure and most of the minerals 

 appeared to be secondary. Chemical analysis showed the rock 

 to have been an andesite. A specimen from a third dyke was 

 generally similar, but was rather less distinctly foliated. 

 A somewhat similar, but rather larger intrusive mass by the 

 side of the Lago Bianco showed more actinolite and signs of 

 primary felspar, with other minerals. Here the rock retained 

 some likeness to a diabase. The resemblance of certain of these 

 rocks to somewhat altered sediments is remarkable. The 

 author considered the bearing of this evidence upon other and 

 larger masses of "green schist" which occur in the Alps, and 

 expressed the opinion that their present mineral structure may 

 be the result of great pressure acting on more or less basic 

 igneous rocks. — The Waldensian gneisses and their place in the 

 Cottian sequence, by Dr. J. Walter Gregory. The lower part 

 of the sequence of the Cottian Alps has been universally divided 

 into three series, of which the lowest has been regarded as a 

 fundamental (basal) Laurentian gneiss. It was the o ject of the 

 present paper to show that this rock is really intrusive in cha- 

 racter and Upper Tertiary in age. The writer endeavoured to 

 prove this by the following line of argument :—(i) The gneiss 

 consists of only isolated outcrops instead of a continuous band, 

 and these occur at different positions and not always at the base 

 of the schist series; (2) the gneiss is intrusive, because (a) it 

 includes fragments of the overlying series instead of vice versa, 



(b) it sends off dykes of aplite into the surrounding schists, 



(c) it metamorphoses the rocks with which it is in contact, and 

 ((/) the schists are contorted near the junction ; (3) the gneisses 

 were further shown to be later than the igneous rocks intrusive 

 into the " pietre verdi " series, as these never traverse the 

 gneiss. No positive opinion as to the age of the overlying 

 schists was expressed, though it was pointed out that the recent 

 discovery of radiolarian muds in the series may necessitate their 

 inclusion in the Upper Palceozoic. The freshness of the gneisses, 

 the fact that these have not been affected by the early Tertiary 

 earth-movements, and the absence of authentic specimens of the 

 gneiss in the Cretaceous, Eocene, and Miocene conglomerates, 

 renders their late Tertiary age highly probable. The nature of 

 the contact-metamorphism and the origin of the gneissic structure 

 were discussed, and a classification offered of the earth-move- 

 ments in the Cottian Alps. A discussion followed, in wtiich the 

 President, Prof. Judd, Mr. Barrow, Prof. Bonney, Mr. A. M. 



