Mr. W. C. Roberts on certain Alloys of Silver and Copper. 553 



where there are several positive and several negative electrodes, 

 r . r . . . &c. being measured from the points where the electrodes 

 cut the plane of the section. 



Hence the forms of these equipotential curves are the same as iu 

 a plane sheet ; so that the forms traced out in tinfoil will be the 

 same as the corresponding forms in space for line electrodes. 

 These forms may be traced out in sulphate of copper with copper 

 electrodes, or in sulphate of zinc with amalgamated zinc electrodes ; 

 and for these experiments, with cylindrical and other vessels, the 

 polar coordinates may be measured directly. One of the battery- 

 electrodes is made the origin of coordinates, and a lath, or brass 

 wire, resting on the edges of the vessel has a slot along it, the 

 origin being at one end or at some point of the slot. In the slot 

 is a sliding piece of wood or ivory which carries one of the galvano- 

 meter-electrodes, and the lath is capable of turning about the 

 battery-electrode on which it is placed. Around this electrode is 

 a graduated circle for measuriug the angles about the origin ; and 

 on the sides of the slot is a millimetre-scale for measuring the 

 distances from the origin. 



The other galvanometer-electrode may be fixed in a manner 

 which is most suitable in each case. 



The results of these investigations show how closely the experi- 

 mental determination of equipotential surfaces and lines of force 

 agrees with the theory of electrical distribution in space. 



March 18. — Joseph Dalton Hooker, C.B., President, in the Chair. 



he following communication was read : — 



" On the Liquation, Fusibility, and Density of certain Alloys of 

 Silver and Copper." By W. Chandler Roberts, Chemist of the 

 Mint. 



The author states that the most remarkable physical property of 

 silver-copper allo} r s is a molecular mobility, in virtue of which cer- 

 tain combinations of the constituents of a molten alloy become 

 segregated from the mass, the homogeneous character of which is 

 thereby destroyed. These irregularities of composition have long 

 been known, and reference is made to them in the works of 

 Lazarus Erckern (1650) and of Jars (1774). A very complete 

 memoir was published in 1852 by Levol, who did much towards 

 ascertainiug the nature and defining the limits of this molecular 

 mobility. He discovered the important fact that an alloy containing 

 71' 89 per cent, of silver is uniform in composition. Its chemical 

 formula (Ag 3 Cu 2 ) and peculiar structure led him to conclude that 

 all other alloys are mixtures of this, with excess of either metal. 



The electric conductivity of these alloys was studied in 1860 by 

 Matthiessen, who doubted the accuracy of Levol's theory, and 

 viewed them as "mechanical mixtures of allotropic modifications of 

 the two metals in each other." 



The author then described the experiments he made with a 



