August i i, 1898] 



NATURE 



357 



electrical resistance, and the following extract from Lord 

 Rayleigh's note gives his suggested explanation on the sup- 

 position that the metals in an alloy are arranged in laminse, and 

 that the current flows across the lamina*. 



"According to the discovery of Peltier, when an electric 

 current flows from one metal to another there is a development 

 or absorption of heat at the junction. The temperature dis- 

 turbance thus arising increases until the conduction of heat 

 through the laminae balances the Peltier effect at the junctions, 

 and it gives rise to a thermo-electromotive force opposing the 

 passage of the current. Inasmuch as the difference of tempera- 

 ture at the alternate junctions is itself proportional to the 

 current, so is also the reverse electromotive force thereby called 

 into play. Now a reverse electromotive force proportional to 

 current is indistinguishable experimentally from a resistance; 

 so that the combination of laminated conductors exhibits a false 

 resistance, having (so far as is known) nothing in common with 

 the real resistance of the metals." 



The structure of eutectic alloys seems to have a special 

 bearing on this question, and seems to afford strong support to 

 the view suggested by Lord Rayleigh, Guthrie pointed out 

 in 1884 that the particular alloy of two metals possessing the 

 lowest freezing point of any alloy of the two that can be made, 

 and which he called the eutectic, is analogous to a cryohydrate, 

 the cryohydrates being regarded as eutectics of ice and the 

 particular salts employed. 



Fig. I. — Silver-lead eutectic, X loo. Oblique illumination. 



As Prof. Roberts-Austen in his valuable Cantor Lectures on 

 Alloys (delivered March-April 1897) has pointed out, the 

 analogy between cryohydrates, eutectic alloys and the pearlite 

 of steels is now completely established. The elaborate micro- 

 scopical investigations of steel and of eutectic alloys made by 

 Osmond, Charpy, Stead and others, together with the work 

 of Ponsot on the cryohydrates, reveal the presence in each 

 case of two different constituents arranged in microscopic 

 lamince. In the case of the cryohydrates the two constituents 

 are ice and the salt, in eutectic alloys they are the constituent 

 metals, and in the pearlite of steels they consist of alternate 

 layers of pure iron and iron carbide. 



In connection with an investigation of the micro-structure of 

 silver-lead alloys the writer has had occasion to examine the 

 eutectic of these two metals, an alloy containing about 2*8 per 

 cent, of silver, and the accompanying photographs of this convey 

 an excellent idea of the structure of eutectic alloys in general. 



Fig. I shows the appearance presented by a polished surface of 

 a section of this alloy after etching for several hours with acetic 

 acid at the ordinary temperature. The lead has partially dis- 

 solved, exposing the silver in bright plates, the edges of which, 

 a good deal bent over and distorted by the action of the stream 

 of wash water, are presented to the observer. A section cut at 

 right angles to the one figured, which is cut parallel to the 

 cooling surface, presents a similar appearance. 



By acting on a portion of this alloy with the vapour of hot 

 acetic acid for several weeks the lead was wholly dissolved, and 

 the bright plates were separated and examined. They proved 

 to be pure silver. They are translucent, the light transmitted 

 through them being violet or greyish violet. Some of these 

 plates were mounted in balsam, and Fig. 2 is reproduced fronv 

 a photograph of one such preparation taken with a ^' oil 

 immersion objective. Measurements of a number of plates 

 which happened to be lying on edge showed that their thickness 

 was less than TrrriuTr of an inch, but accurate measurements in 

 this way are not possible owing to the " black and white dot"" 

 effect well known to microscopists. 



As will be seen from the figure, the plates exhibit distinct 

 cleavage at angles pf 60° or 120° to their longer axes. Some 

 of them are seen to be crossed by a series of faint markings at 

 these angles, markings bearing a very curious resemblance to- 

 those obtained by Commander Hartmann by subjecting metallic 

 plates to compressional or torsional strain (Hartmann : " Dis- 

 tribution des deformations dans les metaux soumis a des 

 efforts," Figs. 21 and 173, pp. 25 and 175). It is difficult to 

 avoid the conclusion that they have a similar origin, the straits 

 in this case being probably due to the shrinkage of the alloy on- 

 solidification or on subsequent cooling. A distinct folding or 

 crumpling of the plate can be seen in the photograph, showing 

 that in spite of their pronounced directions of cleavage the 

 plates are not excessively brittle. 



The bearing of this structure of an alloy on Lord Rayleigh's- 

 remarks will be readily understood. The greater number of 

 alloys which have been subjected to tests of their electrical re- 



NO. 1502, VOL. 58] 



plate, X 3007 



sistance are partially made up of the eutectic of their con- 

 stituents, the remainder of the alloy consisting of one of the 

 two metals or of a compound of the two. It is not conceivable 

 that the work done in rolling and wiredrawing, though it may 

 cause some splitting up of the plates in the eutectic, should 

 entirely destroy this laminated structure ; and its existence 

 would almost certainly give rise to the thermo-electric effects 

 which may be the cause of the abnormal resistance of many 

 alloys compared with that of the metals of which they are. 

 composed. Saville Shaw. 



THE BOARD OF EDUCATION BILL. 



'T'HE following are the clauses of the Bill introduced by the 

 ■*■ Duke of Devonshire in the House of Lords last week, 

 and having for its object the establishment of a Board of Educa- 

 tion for England and Wales. 



I. — (i) There shall be established a Board of Education 

 charged with the superintendence of matters relating to education 

 in England and Wales. 



(2) The Board shall consist of the Lord President of the 

 Council, Her Majesty's Principal Secretaries of State, the First 

 Commissioner of Her Majesty s Treasury, the Chancellor of Her 

 Majesty's Exchequer, and one other person appointed by Her 

 Majesty the Queen and holding office during Her Majesty's, 

 pleasure, and it shall be lawful for Her Majesty to appoint a. 



