35S 



KNOWLEDGE. 



September. 1911. 



the surface tension due to gradual solution or to local cooling 

 as the case may be. It is well shown by a drop of 

 orthotoluidine containing paratoluidine. 



Spherical drops of a liquid are sometimes formed dming a 

 distillation on the surface of the liquid which is being evapor- 

 ated. This, however, is a different affair, the retention of the 

 spherical form being probably due to some difference in 

 composition and hence surface tension of the liquid and the 

 drop. 



Mr. Darling's experiments which have been recently 

 described in " Knowledge " show most beautilully the 

 gradual formation of a drop, which Professor Worthington 

 has studied on a smaller scale photographically. Mr. 

 Darling has devised a method of preserving a sphere of 

 one liquid in another of equal density. A beaker is filled 

 to two-thirds of its height with water at 22" C. : one 

 hundred centimetres of a three per cent, solution of 

 common salt are discharged at the bottom of the 

 beaUer. making thereby the lower layers of liquid slightly 

 denser. .\ large tap funnel containing orthotoluidine at about 

 20" is inserted so that the stem of the funnel is seven centi- 

 metres or so from the bottom of the beaker. By slowly 

 opening the tap the orthotoluidine will form a sphere of quite 

 eight centimetres in diameter suspended within the water and 

 can be shaken free from the stem. 



Sulphur when heated on a glass slip, gathers itself into a 

 flat bottomed drop and does not wet the glass, residing on it 

 like a drop of mercury though not quite such a spherical drop 

 could be formed. The drop will solidify in the same shape 

 as it forms as a drop, but above 256" it begins to flatten and 

 '■ wet " the surface of the glass. The description of these 

 phenomena is due to Mr. W. A. D. Kudge. 



In Mr. Darling's experiments on the formation of drops, as 

 the large sphere of aniline hits the bottom of the beaker it 

 flattens out into a disc-like form with rounded edges, in fact 

 the aniline at the bottom of the beaker takes up a shape 

 something like the small drop of sulphur in Mr. Kudge's 

 experiment. 



There are many phenomena of a curious and interesting 

 nature connected with surface tension which it is well to call 

 attention to from time to time as their explanations are as yet 

 incompletely worked out. 



ALLOTROPV. — Many substances can exist in more than 

 one form ; there are at least three distinct " allotropic " forms 

 in which sulphur can occur, or two in which phosphorus 

 exists, to take well-known examples. Thus sulphur exists in a 

 rhombic crystalline form melting at 112-8° C, and in a 

 monoclinic form melting somewhat higher, and being of a 

 browner hue and insoluble in carbon bisulphide. Further 

 there are other less distinct forms, such as colloidal sulphur 

 formed by precipitation of sulphur from thiosulphates bv 

 strong acids, and so on, or plastic sulphur which is an 

 extensible gummy brownish-yellow mass formed by suddenly 

 cooling liquid sulphur, and is the latter in a supercooled state, 

 which changes fairly rapidly into rhombic sulphur. .\ peculiar 

 gummy sticky variety of plastic sulphur rather different in 

 many respects to the other variety can be made by suddenly 

 cooling liquid sulphur by pouring it into liquid air. This 

 variety changes very rapidly, especially on touching it, into 

 very yellow rhombic sulphur. 



Much work on allotropy and the ei|uilibrium between the 

 different forms of substances under different conditions of 

 temperature and pressure, has been done by Professor Cohen, 

 of Leyden, and his collaborators. Smits has recently published 

 a theory to explain the phenomenon of allotropy. In the 

 litjuid state a substance showing allotropy consists of two 

 kinds of molecules ; the number of one kind existing in 

 presence of the other depends on the temperature, and, at any 

 particular temperature, there exists an equilibrium between the 

 two kinds. On rapid cooling, all of the one form does not 

 change as the temperature falls, into the simpler form which is 

 stable at the lower temperatures, because the equilibrium 

 cannot keep pace with the change in temperature ; solidification 

 then occurs at a temperature differing from the solidification 



point of one of the molecular forms. Thus if rhombic sulphur 

 be melted and allowed to cool to 90° C. so that equilibrium 

 sets in at this temperature and then is suddenly cooled, the 

 resulting sulphur melts at 110-9' C or, if equilibrium occurs 

 at 65°C. the melting point is 111-4°C. This points to the 

 existence of two kinds of molecules. In the case of phosphorus 

 which in the pure crystalline state, melts at 44° C. + -02° C, 

 the melting point is considerably affected by previous heating at 

 different temperatures, pointing to at least two different forms 

 of molecules being present ; the violet form, which is stable 

 below 460° C can be best accounted for by a discontinuity of 

 the series of mixed crystals of the yellow and red forms. 

 -Mercury exists in only one modification, which is in agreement 

 with its single molecular condition adduced from other con- 

 siderations. Tin and many other elements exist in more than 

 one form. The theory explains readily the phenomena of 

 undercooling and superheating such as occur during rapid 

 cooling and heating. Having mentioned superheating it is 

 interesting to note that the vapour pressure curve of solid 

 bromine, obtained recently by Cuthbertson by optical means, 

 shows evidence of superheating of this solid, the vapour pres- 

 sure being greater than that of the liquid above the real melting 

 point of the solid. 



LIQUID HELIUM. — Kamerlingh Onnes has been making 

 further experiments with liquid helium ; he finds the density 

 at 4-29° from the absolute zero ( — 273-12-C.) to be 0-122 

 and the liquid appears to possess a maximum density at 2-2°. 

 He has also found that mercury when exposed to a tempera- 

 ture of 3 absolute, such as is produced by hquid helium 

 boiling under reduced pressure, has an exceedingly great 

 electrical conductivity : ten miUion times greater than at O-C; 

 which is rather a surprising result, considering that from the 

 temperature coefficient one would expect a resistance of 

 40 ohms, instead of 3 X 10"'' ohms ! Other properties, such as 

 magnetic susceptibility, are considerably altered by these very 

 low temperatures, though the electrical resistance of other 

 metals, such as silver, lead, eureka, and so on, is not modified 

 to anything like the same amount as that of mercury. Further 

 experiments to elicit the cause of the phenomena will be 

 awaited with interest. 



UK-iVNIUM. — The name of Becquerel has been associated 

 with the study of phosphorescence for generations. M. J. 

 Becquerel has been studying for some time the effect of low 

 temperatures on phosphorescence which, in general, causes an 

 increase in the duration of the phosphorescence. However, he 

 finds that the low temperature does not affect the duration of 

 the phosphorescence of uranium nitrate and sulphate — the 

 most phosphorescent of uranium salts : while the acetate, 

 tartate, and so on, show nuich longer persistence of phosphor- 

 escence at low temperatures. Uranium nitrate exhibits 

 marked " triboluminescence " ; if a bottle of the nitrate be 

 shaken in the dark, flashes of light will be seen ; this is due to 

 cleavage of the crystals accompanied probably by minute 

 electrical discharges which excite the crystals to phosphoresce. 

 It is a very different phenomenon to the sparks of uranium 

 obtained by shaking a bottle of metallic uranium. The latter 

 is a " pyrophoric " property ; the uranium particles which are 

 struck off in the shaking actually burn in the air, owing to the 

 avidity with which uranium combines with oxygen ; the same 

 phenomenon is found with cerium and iron. I have been able 

 to run a petrol engine using these sparks as the igniting agent 

 for the explosive mixture, but I do not think the method a 

 practical one owing to the injury to the cylinder walls. The 

 triboluminescence of uranium salts is a very different affair 

 from the pyrophoric property of the metal : the tribolumin- 

 escence is unaffected by an atmosphere of hydrogen, while 

 the pyrophory no longer occurs. 



Pitchblende is the main ore from which uranium is 

 obtained; it is also the chief source of radium. It is 

 noteworthy that H. Poole has found that the evolution of heat 

 by a mass of pitchblende is greater than would be expected 

 from its radium content, and the fact that radium generates 

 one hundred and ten calories per hour. The difference 

 between the observed heat generation and the calculated 



