Hydro- Electrical Currents. 99 



1 per cent, in density, this process was repeated fifty successive times, 

 without any change in the specific gravity, when tested in a soft state. 



Antimony and bismuth bars, when often heated and immersed in cold 

 water, lose density on each repetition of the heating and immersion ; 

 they become hollow in the centre, and full of fissures ; zinc bars con- 

 tinue solid under the same treatment. The following metals lose density 

 by annealing : — Bismuth, zinc, lead, tin, copper, silver, iron, gold, pla- 

 tinum. Antimony cast in a cold mould, and hard steel, gain density by 

 annealing. Zinc, cadmium, lead, and tin, gain density when poured on 

 ice, compared with the same pieces poured on a flat surface, and slowly 

 cooled. Antimony and bismuth shewed no change in density when 

 treated like zinc, cadmium, &c. ; but when cast in a confined cold 

 metal mould, in bars similar to those used for battery, fig. 1, they lose 

 about s^^th of their specific gravity. Copper and silver lose density when 

 poured on ice, or on cold metal surfaces. 



In these experiments, bismuth and antimony resemble hard steel, in 

 so far as the process of hardening is concerned ; but in the annealing, 

 antimony only follows the same change as the steel, and gains density. 

 Bismuth, when cast in a metal mould immersed in dry snow, loses^^V part 

 of its density in the lower portion of the bar, yet a still further loss of 

 about T^g part is produced by annealing ; while antimony, cast under 

 similar circumstances, suifers the same loss of density ; but with annealing 

 always regains part of the loss. 



In comparing the changes in the densities of the metals for equal in- 

 tervals of temperature, in the foregoing experiments, bismuth passes 

 through the widest range of variations, then follow zinc and antimony. 



32. Experiments with antimony and bismuth couples were now made, 

 to ascertain if, while they were engaged to develope thermo-currents, 

 the same changes in density took place ; the mean density of three speci- 

 mens of bismuth changed during twenty-one days' action from S.G. 9.853 

 to 9.838, three specimens of antimony for the same period S. G. 6.645 to 

 6.670. The direction of these changes agree with the former experiments 

 where the metals are operated on singly. At first sight I expected they 

 would exercise much influence on the energy of thremo-electrical cur- 

 rents developed by antimony and bismuth ; but from a number of clear 

 experiments with bars in difierent states combined in thermo couples, 

 I have found the more nearly the bars approach to their natural density, 

 when in a soft state, the better they are adapted for energetic action in 

 combination with another metal. The experiment which first shewed 

 me this fact, should sufiiciently illustrate the whole of the class. A bis- 

 muth bar, combined with a bar of soft steel, and attached to the galvano- 

 meter, was heated by oil to 95°, the deflection in the galvanometer was 67% 

 The bar of steel was thenuntiedto harden it; after hardening, the arrange- 

 ment was now replaced, to be in every respect the same as before, when 

 the couple of bismuth and hard steel shewed a diminution of 2° in the 

 deflection of the galvanometer needles. This experiment was many times 

 repeated with other elements, all of which agreed in shewing a result 

 similar to the above, and to develope the greatest amount of electricity 

 from a given couple. Both sides must be thoroughly annealed ; hard bismuth, 



