Intelligence and Miscellaneous Articles. 311 



Dr. Bleekrode has varied these experiments considerably. He 

 immersed two electrodes of the same metal in two vessels containing 

 a salt of this metal, united by a siphon. One of these vessels was 

 maintained at nearly a* constant temperature, whilst the other, placed 

 in a hot air-bath, was gradually heated. The electrodes were con- 

 nected by a circuit, including a coil whose resistance was such 

 to render imperceptible the variations produced by heat in the con- 

 ductivity of the liquid ; and in the second place a reflecting galvano- 

 meter, the deflections of which never exceed 3°, measured directly the 

 intensities. As the resistance of the circuit remained sensibly con- 

 stant, these deflections were also proportional to the electromotive 

 forces. 



The memoir by Dr. Bleekrode is very rich in numerical results. 

 These results are collected in twenty-one Tables, of which the majo- 

 rity are represented by curves. Some of these curves do not differ 

 much from the straight line, so that the electromotive force is pro- 

 portional to the difference of the temperatures ; this is what takes 

 place, for instance, with copper in sulphate or nitrate of copper, 

 and with amalgamated zinc in sulphate or chloride of zinc ; but it is 

 rarely like this. 



In general the current goes through the siphon from the cold 

 vessel to the heated vessel, except with silver immersed in acetate or 

 nitrate of silver, when it is in a contrary direction. 



The most curious cases are those in which the current changes 

 sign. Of these the author distinguishes three. When amalga- 

 mated zinc is immersed in a solution of double cyanide of zinc and 

 potassium, the current is at first positive or passes from cold to hot ; 

 then it changes direction when the difference of temperatures exceeds 

 30° ; it is the same at 53° for silver in double cyanide of silver and 

 potassium. Lead in nitrate of lead is still more singular : the couple 

 is at first negative (the current passes from hot to cold) ; then it 

 changes direction when the difference of temperatures exceeds 21°, 

 and again changes and becomes negative when the difference 

 reaches 51°. 



Too much importance must not be attached to the numerical de- 

 terminations of electromotive forces, because the composition of the 

 liquids changes under the action of the current, and soon produces 

 other currents which are superposed on the first and produce hetero- 

 geneity of the solutions in which the electrodes are immersed. This 

 phenomenon is particularly remarkable with silver immersed in ni- 

 trate of silver. Although in this case the current passes from hot 

 to cold, it produces in the heated vessel some free acid at the same 

 time that it deposits some metallic silver, partly black and partly 

 crystallized under an arborescent form. — Poggendorff's Annalen, 

 vol. cxxxviii. p. 571 ; Annates de Chimie, April 1870. 



ON TESTS FOR THE PERFECTION AND PARALLELISM OF PLANE 

 SURFACES OF GLASS. BY WOLCOTT GIBBS, M.D., RUMFORD PRO- 

 FESSOR IN HARVARD UNIVERSITY. 



When a plano-convex lens of long radius of curvature is placed 



