384 Intelligence and Miscellaneous Articles. 



nature of the optical image at which the line terminates. Still 

 there is a manifest advantage in the convention proposed, since it 

 allows the relation of focal length and conjugate focal distances to 

 be represented by a single equation for both mirrors and lenses. 



I give below the adaptation of the graphical method to the 

 convention of signs proposed by Prof. Lodge (and used in Granot's 

 'Physics'), viz. : — Distances to real images to be considered positive 

 and distances to virtual images negative. 



The fixed points through which the rotating lines pass are now 

 in the right-hand upper quadrant both for convex lenses and 

 concave mirrors, and in the left-hand lower quadrant for concave 

 lenses and convex mirrors. 



That is, referring to Pig. 1, page 61, Jj x is now shifted so as to 

 coincide with M a which remains unmoved, and L ffl similarly is 

 made to coincide with M, r . 



Tours faithfully, 



Univ. Coll., Nottingham, Edwin H. Baeton. 



SOLUTION AND DIFFUSION OF CERTAIN METALS IN MERCURY. 

 BY W. J. HUMPHREYS. 



The investigation, of which this is a summary, was begun with 

 the object of determining the extent to which these phenomena 

 differ, if at all, in this case from the solution and diffusion of non- 

 metallic solids and liquids. 



The method of investigation was to fill a vessel of constant cross 

 section with pure mercury, put on its surface a freshly amalgamated 

 piece of the metal to be examined, and after allowing it to stand 

 a definite length of time in a place free from external disturbances 

 and of fairly constant temperature, to remove from known depths 

 below the surface samples of the amalgam and analyse them. 



The metals examined were lead, tin, zinc, bismuth, copper, and 

 silver, and the results indicated that there is no essential difference 

 between the solution and diffusion of these metals in mercury and 

 the same phenomena in any other case. 



Probably the most interesting results were those given by copper 

 and silver, both of which dissolved to a much less extent than any 

 of the other metals examined, but diffused more rapidly. At 28° 

 0. the silver dissolved to the extent of only about one part in two 

 thousand, and the copper to a still less extent — about three parts 

 in a hundred thousand ; while the rate of diffusion of the silver 

 was about twenty millimetres per minute, approximately sixty 

 times that of copper and fully six hundred that of zinc. 



This investigation, of which the details will soon be published, 

 was suggested to me by Dr. J. W. Mallet, P.E..S., of the University 

 of Virginia, and carried out there under his supervision during the 

 months of August and September, 1895. — John Hopkins University 

 Circulars. Pebruarv 1896. 



