June 23, 1892] 



NATURE 



189 



the critical curve, due to the formation of definite atomic com- 

 pounds, AgZn, and AgjZn, ; no corresponding indications are 

 observed when aluminium replaces zinc in these mixtures, 

 whence apparently similar atomic combinations of silver and 

 aluminium are not formed. On the other hand, lead-zinc-tin 

 md lead-aluminium-tin alloys correspond sharply with each other 

 in that they are the only alloys yet examined where the direction 

 of slope of the tie lines is not the same throughout ; in each case 

 the lower ties slope to the left (lead side), and the upper ones to 

 the right (zinc side) ; the point where the angle of slope of the 

 lower ties is a maximum corresponds in each case with a ratio 

 between lead and tin in the heavier alloys formed approximating 

 pretty closely to that indicated by the formula PbjSn, suggest- 

 ing that the sloping of the lower ties is due to the formation of 

 this definite atomic compound. The upper ties in each case 

 exhibit a tendency to converge towards a point on the right- 

 hand side of the curve, approximately where the ratios of zinc 

 to tin and aluminium to tin respectively are those indicated by 

 the formulae Zn4Sn, and Al4Sn ; suggesting the existence of 

 these definite compounds. 



In the course of the experiments, it is shown incidentally that 

 bismuth and aluminium are practically immiscible when molten : 

 at about 900° C. bismuth dissolves less than O'l per cent, of 

 aluminium, whilst aluminium dissolves about 2'0 per cent, of 

 bismuth. The binary alloys of bismuth and aluminium stated by 

 previous observers to exist are simply more or less intimate inter- 

 mixtures of the two metallic solutions. 



"On the Theory of Electrodynamics as affected by the 

 nature of the Mechanical Stresses in Excited Dielectrics." 

 By J. Larmor. 



The various theories of electrodynamics are examined from the 

 standpoint of their ability to explain the experimental facts as 

 to pressures in liquid dielectrics which have been made out by 

 Quincke and other experimenters. 



The principal conclusions are as follow : — 



(i) It follows from the experimental results that the stress in 

 an excited fluid dielectric between two condenser plates consists, 

 at any rate to a first approximation, of a tension along the lines 

 of force and an equal pressure in all directions at right angles 

 to them, superposed upon such stress as would exist in a vacuum 

 with the same value of the electric force. 



(2) It follows from experiments that the numerical value of 

 these additional equal tensions and pressures is, at any rate to a 

 first approximation, (K - i)F-/8ir, where F is the electric force, 

 and K the inductive capacity. 



(3) Such a distribution of equal tension and pressure is 

 necessarily the result of a uniform volume distribution of energy 

 in the dielectric, irrespective of what theory is adopted as to its 

 mode of excitation. 



(4) If we consider the mode of excitation to be a quasi- 

 magnetic polarization of its molecules, the numerical magnitude 

 of these stresses should be 



where A. is a coefficient which depends on the molecular dis- 

 creteness of the medium, and is probably not very different from 

 the value \-n. A discrete polarization of the molecules does not 

 account for the stress, so far as this coefficient is concerned. 



(5) The stress which would exist in a vacuum dielectric is 

 certainly due in part to a volume distribution of energy, as is 

 shown by the propagation of electric waves across a vacuum. 

 There is thus no reason left for assuming any part of it to be due 

 to a distribution of energy at its two surfaces, acting directly on 

 each other at a distance. There is therefore ground for assum- 

 ing a purely volume distribution of energy in a vacuous space, 

 leading to a tension (l/8ir)F2 along the lines of force, and a 

 pressure ( l/8ir)F* at right angles to them. 



(6) The ^Maji-magnetic polarization theory rests on the 

 notion of a dielectric excited by a surface charge on the plates, 

 and therefore involves a surface-distribution of energy, unless in 

 the extreme case when the absolute value of K is very great ; in 

 that case a slight surface-charge produces a great polarization 

 effect, and in the limit the polarization may be taken as self- 

 excited. Thus the absence of a surface-distribution of energy 

 leads to Maxwell's displacement-theory, in which all electric 

 currents are circuital, and the equations of electrodynamics are 

 therefore ascertained. 



(7) It appears that even this limiting polarization theory 

 must be replaced, on account of the stress-formula in (4), by 



NO. II 82, VOL. 46] 



some dynamical theory of displacement of a more continuous 

 character. 



"The Hippocampus." By Dr. Alex. Hill. 



Observations upon the brains of various aquatic maiiimals 

 which show that, when the sense of smell is completely absent, 

 as judged by the total suppression of the olfactory bulb and 

 nerves, the essential constituent of the "hippocampus," the 

 fascia dentata, is absent also. Amongst other mammals the 

 fascia dentata varies in development as the olfactory bulb. 



Chemical Society, June 2.— Dr. W. H. Perkin, F.R.S. 

 Vice-President, in the chair. — The following papers were 

 read :— Ethylene derivatives of diazoamido-.compounds, by R. 

 Meldola and F. W. Streatfeild. The authors find that ethylene 

 diazoamides result from the action of ethylene dibroroide and 

 caustic alkali on diazoamides dissolved in alcohol. The pro- 

 ducts thus obtained have the general formula — 



/^ 



X. /N<; 



^>n/\c,H,; 



\h 



mixed ethylene-diazoamides can also, be obtained. These com- 

 pounds are very stable, and are decomposed by prolonged con- 

 tact with concentrated hydrochloric acid at ordinary tempera- 

 tures. — The action of light on silver chloride, by H. B. Baker. 

 The small loss in weight which silver chloride undergoes on 

 exposure to light has led the author to investigate whether oxygen 

 is absorbed at the same time that chlorine is evolved, Robert 

 Hunt having long ago stated that such an absorption does occur. 

 Silver chloride was placed in a bulb connected with a long tube 

 standing over caustic potash solution ; the atmosphere consisted 

 of air or oxygen. On exposing this apparatus to light, the 

 liquid rose in the tube, showing that oxygen had been absorbed. 

 The proportion in which the elements silver, chlorine, and 

 oxygen were contained in the darkened substance was then de- 

 termined. The results agree approximately with the formula 

 AgoClO. The experimental difficulties in the way of an exact 

 determination of the composition of the darkened product are 

 very great, owing to the fact that the dark substance is mixed 

 with an enormous quantity of unchanged silver chloride. The 

 product of exposure to light never contains more than o'l gram 

 of the dark- coloured substance per 50 grams of unaltered silver 

 chloride. Another cause of inaccuracy complicates the result if 

 the analyses be not made immediately after exposure ; the 

 darkened substance turns white again if left in the dark, 

 probably owing to the formation of another oxychloride. The 

 thoroughly dried darkened substance yields water on reduction 

 in pure hydrogen, and also ^evolves oxygen on treatment with 

 pure chlorine. The results of determinations of the constituents 

 of the darkened product made by various analytical methods 

 agree as well as could be expected.' Silver chloride does not 

 darken when exposed to light in absence of oxygen. No 

 darkening was observed in a vacuum, in carbon dioxide, or in 

 carefully purified carbon tetrachloride. It is, however, difficult 

 to eliminate traces of alcohol, carbon disulphide, &c., from 

 carbon tetrachloride, and the darkening caused by these 

 impurities has hitherto been confounded with that caused 

 by oxygen. Darkened silver chloride dissolves completely 

 in boiling potassium chloride solution, and the solution 

 so obtained contains caustic potash. The production of this 

 free alkali seems to prove conclusively that oxygen is present 

 in the darkened substance in the combined state. — The estima- 

 tion of slag in wrought iron, by A. E. Barrows and T. 

 Turner. Cast iron comparatively rich in non-metallic elements 

 is known to afford a greater yield of puddled bar than do purer 

 samples of iron. The loss on reheating and rolling into finished 

 iron is, however, also greater. This difference has been attri- 

 buted to intermingled slag. The authors prepared four samples 

 of iron, viz. best bar, best sheet from the same bar, common 

 bar, and common sheet from the same. Pig iron of known 

 composition was used ; the yield of common puddled bar was 

 65 per cent, greater than that of the other, but the loss on 

 reheating was also i "5 per cent, greater, leaving a balance of 

 5 per cent, in favour of the common iron. The results of analyses 

 show that the silicon is equally, and very slightly, reduced in 

 each case, whilst the phosphorus was much reduced in common 

 iron, and but scarcely affected in best. This does not favour the 

 view that much more slag is removed in one case than in the 



