564 Intelligence and Miscellaneous Articles. 



conductivity and diffusion of an electrolyte. — Zeitschrift fur 

 physikcdische Ohemie, vol. vi. p. 564 ; Beibldtter der Physik, vol. xv. 

 p. 370. 



METHOD OF DETERMINING THE SURFACE-TENSION OF MERCURY. 

 BY H. SENTIS. 



A rectangular plate of iron of the volume abc and the weighty 

 floats on mercury ; the actual dimensions were about 120 X 8 X 2 

 c. millim. The depth h to which it siuks is determined by means 

 of a spherometer. On the one hand, we have from the principle 

 of Archimedes, 



jp=ab7iD + 2(a + 6)F cos a ; 



in which D is the density of mercury, F the surface-tension, and 

 a the edge-angle, and the second term on the right is the weight 

 of the mercury which would fill up the groove about the plate. 

 The error which is due to the corners may be eliminated by 

 means of another plate of volume cde, and weight p\ immersed 

 to the same extent, which gives exactly the same error. On the 

 other hand, the well-known equation 



/2I 

 V D 



(1— cos a) 



holds, and by eliminating the error and the edge-angle we get the 

 formula 



4 + [d+b-c-dJK 2 !) ' 



from which observations give F=39'23 mg. — Journal de Physique, 

 vol. ix. p. 384 (1890). 



ALLOTROPIC SILVER. 



Mr. M. Carey Lea requests us to correct a typographical error 

 occurring in his paper, entitled "Notes on Allotropic Silver," 

 which appeared in our October number. In this paper in several 

 places appears " protochloride " where photochloride is the correct 

 reading. Silver photochloride is the name proposed some years 

 ago by Mr. Lea for the coloured compounds resulting from the 

 union of normal silver chloride with small quantities of subchloride 

 in no definite proportion, but after the manner of lakes as specially 

 described by him.^ — The violet substance resulting from the expo- 

 sure of silver chloride to light is a photochloride. 



