NA TURE 



[June 25, 1896 



mixture to possess, there would obviously be interference pro- 

 duced by the thermo-electric currents set up owing to the 

 juxtaposition of these masses of unlike metals. It would be 

 parlicularly interesting to observe whether pure entectic alloys 

 or chemical compounds of metals, which alone appear to form 

 really homogeneous masses of mixed metals, would behave like 

 ]nire metals, or like other alloys in regard to conductivity at low 

 temperatures. 



In order to delect the presence of opposing E.M.F.s, we first 

 tried the experiment of passing a fairly intense current through 

 a bar of alloy, breaking contact by means of a simple switch, 

 and immediately making connection with a galvanometer, the 

 interval of time elapsing between the reversal of connections 

 being but a small fraction of a second. We have hitherto, 

 however, failed to obtain any indication in this manner, a fact 

 which we ascribe to the rapidity with which the temperature of 

 the mass is equalised, osving to the minute size of its constituent 

 particles, and to the appreciable time that must elapse between 

 the break of the current and the contact with the galvanometer. 

 Even when the time was reduced to i/iooth of a second or less 

 by the use of a Morse key, and when the process was repeated 

 five times a second, no consistent indication of a residual E. M.F. 

 could be detected with a D'Arsonval galvanometer giving a 

 deflection of 0'5 mm. per micro-volt. The ordinary thermo- 

 electric effects had of course to be eliminated, and were very 

 troublesome. 



The next experiment was to balance the resistance of two 

 pieces of wire of equal diameter — one of copper, the other of 

 alloy — against each other, using equal ratio arms on a Wheat- 

 stone bridge, and then by means of a secohmmeter to try if the 

 resistance of the alloy diminished when the current was rapidly 

 reversed. In all cases there was found to be a distinct reduc- 

 tion of the resistance of the alloy relatively to the copper, the 

 above-mentioned galvanometer giving deflections of from lo to 

 15 mm., with 60 reversals per second. In one case the alloy 

 was copper = 75 per cent. : gold = 25 per cent. , the wire being 

 07 mm. in diameter and 820 mm. long, whilst the copper wire 

 \\as 07 mm. in diameter and 4100 mm. long ; the current used 

 was about i '5 amp., and the resistance of each wire was about 

 o'lS ohms. A deflection of 15 mm. on the galvanometer scale, 

 corresponding to 30 micro-volts, would be caused by an un- 

 balanced E.M. F. in one of the wire = 105 micro-volts, or, say, 

 l/26ooth of the total E. M.F. 



The time actually occupied in the reversal of the current was 

 estimated as varying between l/50oth and l/ioooth of a second, 

 which was the time available for equalisation of temperature 

 before the current was started in the opposite direction. Any 

 residual E. M.F. would then assist the current at " make," and 

 so reduce the apparent resistance. In any case, at the first 

 instant of starting the current, the opposing E.M. F. would be 

 absent, but the time required for its appearance would be very 

 small compared with the time between reversals ( i/6oth of a 

 second). Increased efl'ect should therefore be sought by in- 

 creasing the number of reversals per second rather than by 

 shortening the time between stopping and re-starting the 

 current. 



Care was, of course, taken to ensure equal capacities and in- 

 ductances in each pair of arms of the bridge ; and an experi- 

 ment was made to see if the observed efl'ect was due to the 

 current in the copper wire becoming concentrated near the 

 surface. A copper wire 1000 mm. long and 0'35 mm. in 

 diameter was balanced against another copper wire 4000 mm. 

 long and 07 mm. in diameter; but no variation of resistance 

 was observed. .At this point the investigation was allowed to 

 rest, pending the construction of special apparatus and the 

 arrival of the summer vacation. Waltur G. McMillan. 

 ROISKRT H. IIOUS.MAN. 



Departments of Metallurgy and of Physics, 

 Mason College, Birmingham, June 22. 



Are Rontgen Rays Polarised? 

 Mr. L. C.\sella has made for me a Crookes' tube having 

 as the anode a platinum window sealed into the end of the tube 

 opposite the kathode, which is the ordinary aluminium disc. 

 Owing to the glass sealing, only a small portion of the platinum, 

 about 3 nmi. in diameter, is free to act. The light from all but 

 this portion was screened off by thick glass discs and a brass 

 disc, tliese having e.ach an aperture in the centre. The result, 



NO. 1391, VOL. 54] 



with the fluorescent screen, was at first poor, because the vacuum 

 was too low ; but as that got higher it improved, and I was 

 able to electrograph a part of the hand, by the rays given off by 

 this small platinum window, in 15 seconds, the plate lieing 2i" 

 from the window. An ordinary focus tube takes 30 seconds to 

 produce the same effect under similar conditions, but gives better 

 definition. With the platinum window tube, though the bones 

 are defined on the fluoresceiit screen, there seems to be too 

 much white light, and the difterence between bones and flesh is 

 less marked. The tilted platinum of a focus tube, apparently, 

 reflects most of the kathode rays, but transmits some. Compare 

 the behaviour of the platinum in both tubes with the action of 

 light on glass. With both glass and platinum, part of the rays 

 are transmitted and part reflected, the proportion varying with 

 the angle of incidence ; but, with both, those rays which are 

 perpendicular are apparently transmitted. If the glass be tilted 

 at the proper angle, the reflected rays and a small part of the 

 transmitted rays are polarised. Suppose the plate of glass in 

 the position of the platinum window, and the source of light a 

 luminous point within the tube ; although most of the transmitted 

 light would be radiated direct from the luminous point, part 

 would be rays which had been polarised by reflection from the 

 walls of the tube. The analogy would still hold good, for we 

 know that, as far as X-rays are concerned, glass behaves very 

 similarly to platinum, for these rays are under suitable con- 

 ditions given off by both. 



These considerations and the appearance of microscopic pre- 

 parations containing bone undecalcified when examined by low 

 powers and ordinary light under Nicol prisms, lead me to 

 hazard the suggestion that a bare possibility exists of X-rays 

 being polarised kathode rays. Were this so, the two kinds of 

 X-ray described by several observers would be explained, and 

 we should also understand why those who have tried to polarise 

 these X-rays should have failed, the rays being already polarised. 

 If this view is correct, extinction of the X-rays should be caused 

 by reflection from a second platinum surface at the proper angle. 

 Whether this would succeed at atmospheric pressure, I know 

 not ; the experiment should be tried in vacuo, and a tube con 

 structed specially for the purpose. The window tube has, at 

 all events, proved that a quantity of kathode rays, with some 

 X-rays, may be transmitted through moderately thin platinum 

 under these conditions. J. Willia.m Gifi-'ORD. 



Chard, June 9. 



A Curious Bird's Nest. 



.\ CURIOUS bird's nest, or rather its adjuncts, has lately been 

 presented to the Warwick Museum. It was found in a curved 

 iron pipe intended to deliver water from a well at the baths, 

 and appears not to have been used for some time. The entire 

 length of the pipe was four feet, and the diameter five inches. 

 The bird had built its nest in the centre, and had not only 

 surrounded it with moss and other materials, but had extended 

 them for some length on each side, the total amounting to two 

 feet two inches. The singular thing is that the bird should 

 have taken so much trouble to do this, and it really might seem 

 as if, like the bower birds, it bad done it in sport ; for it was 

 not necessary (though the sharp little bird may have thought 

 so) for the preservation of the nest to extend it so far on each 

 side with moss, feathers, and other things. The eight small 

 eggs in the nest appear to have belonged to the blue titmouse. 



June 22. r. B. Brodie. 



" Hydrodictyon reticulatum." 



It may interest readers of Nature who collect fresh-water 

 Alga;, to know how easy of cultivation is tliis beautiful species 

 It occurs frequently, though by no means every year, in one of 

 the tanks in the Royal Gardens, Kew. Last summer, about 

 this time, I gathered it there in considerable quantities, .\fter 

 preparing specimens for demonstration, I placed the remainder 

 in a gl.ass dish, where it remained in my study, entirely neglected, 

 except for an occasional renewal of the water, until a week or 

 two ago, when I found it still in beautiful condition, with both 

 large and small nets. It is to be found again this year in its 

 original habitat, with nets of an unusually large, almost 

 gigantic, size. .\LFKEt) W. Bennett. 



London, June 20. 



