of Electrolytic Decomposition. 47 



placed, and which seems sufficiently important to merit a few 

 words of description. These porous pots were 2 inches long, 

 £ inch in diameter, and varied from -^q inch to -J thick in the 

 walls. My attention was first attracted to their behaviour by 

 noticing that when they -were partially dipped into fused 

 mercuric iodide^ that liquid rapidly made its appearance in 

 the pot; and the subsequent analysis of the substance sur- 

 rounding the porous pots after an experiment, showed the 

 presence of small quantities of the products of the decompo- 

 sition effected by the current, such as iodine, mercuroso- 

 mercuric iodide, and mercurous chloride. Control experi- 

 ments with water and with fused plumbic chloride showed 

 that these liquids were unable to penetrate the walls of the 

 porous pots. 



The explanation of the facility with which fused mercuric 

 iodide penetrates the walls of a porous pot and rises within it 

 seems to be of a complex nature. The imbibition of this fused 

 substance in the porous walls of the pot is due to capillary 

 action, and does not account for the liquid filling the pot; for 

 since it is of a capillary nature, this action must cease as soon 

 as the inner surface or wall becomes wetted. In the case of a 

 volatile liquid such as fused mercuric iodide this action may be 

 somewhat prolonged by its volatilization from the inner surface 

 of ihe pot- wall, and by the direct formation of crystals from the 

 vapour. The subsequent fusion of these crystals will account 

 for the presence of some liquid mercuric iodide within the 

 porous pot, but then this action must cease. Moreover this 

 explanation apparently requires a difference of tempera- 

 ture within and without the porous pot, which, from some 

 special experiments made upon the subject, can scarcely be 

 assumed to exist ; and I therefore think that the explanation 

 of the penetration of the liquid through the walls of the porous 

 pot must be mainly sought in an easy transpiration of the fused 

 mercuric iodide through its pores in consequence of the small 

 initial difference of level ("head") of the liquid. 



I have already stated that analysis showed the presence of 

 some of the products of decomposition formed within the 

 porous pots in the undecomposed substance in which the latter 

 were partially immersed ; and this seems attributable to diffu- 

 sion through the pot-walls. Little seems known respecting 

 the rates of diffusion of fused substances through porous dia- 

 phragms ; but the particular difficulties in the way of their 

 determination for such volatile substances (which may so 

 readily mix by distillation) seems of itself suggestive of a 

 molecular activity not unconnected with a long free mean 

 molecular path and of rapid diffusion. 



