On the Expansion produced by Amalgamation. 327 



We have to express our thanks to Messrs. Chatterton, 

 Humphrey, and Martin, three of the students of the Central 

 Institution, for assistance rendered in the carrying out of this 

 experiment. 



XXXIX. On the Expansion produced by Amalgamation. By 

 Professors W.E.Ayrton,^.^.^., and John Perky, F.B.S.* 



ON amalgamating the edge of a brass bar, nearly three 

 quarters of an inch thick and about a foot long, for the 

 purpose of enabling the edge to make good electric contact 

 with a plate, we were surprised to find that the bar rapidly 

 curved, the amalgamated edge becoming convex, exactly as 

 happens when one side of a piece of paper is wetted. On 

 hammering the bar to straighten it, the curvature became 

 instead greater. Seeing that to bend a short brass bar more 

 than half an inch in thickness to the extent produced by the 

 amalgamation of the edge requires the exertion of very con- 

 siderable stresses, it follows that very great forces must be 

 produced by amalgamation. 



We think it possible that this bending by amalgamation 

 may be an important cause in the production of the Japanese 

 " magic mirrors/' the reflecting surface of which is polished 

 with a mercury amalgam. Japanese mirrors are made of 

 bronze and have a raised pattern cast on their backs ; and 

 although the eye can detect no trace of the pattern on looking 

 at the polished reflecting surface, yet when certain of these 

 mirrors are used to reflect a divergent beam on to a screen, 

 the pattern at the back can be seen as a bright image on a 

 dark ground. In a paper communicated, some years ago, to 

 the Royal Society, we showed that this peculiar effect arose 

 from the fact that, while the reflecting surface was generally 

 convex, the portions corresponding with the pattern or thicker 

 parts were less convex (that is, more concave) than the rest ; 

 and this conclusion we verified by finding that when a conver- 

 gent, instead of a divergent, beam of light was allowed to fall 

 on the mirror the image on the screen was reversed ; that is, 

 the pattern was seen as a dark image on a bright ground 



This inequality of curvature we considered at that time was 

 due partly to the pressure of the " distorting-rod " used to 

 make the surface convex, and partly to the pressure exercised 

 on the subsequent polishing ; but we now think that, in ad- 

 dition, the action of the mercury-amalgam employed by the 

 polisher may assist in making the thin portions of the mirror 

 more concave than the thicker. 



Communicated by the Physical Society : read March 27, 1886. 



