500 Prof. Williamson on Lepidodendra and Sigillarise. [Julie 15, 



The amount of action in a circuit of two metals and a saline solution 

 must have as one of its regulating conditions the conducting-power of that 

 solution. It appeared by experiment that a strong solution of nitrate of 

 silver offers less resistance than a weak one ; and it was also found, on 

 adding nitrate of potassium to the nitrate of silver, that its power of 

 attacking the copper plate was increased, that the augmentation of the 

 foreign salt increased the action still further, and that the 2-3 law holds 

 good between two solutions in which both the silver and potassium salt are 

 doubled, though it does not hold good if the quantity of foreign salt be kept 

 constant. Similar results were obtained with mixed nitrates of silver and 

 copper. 



While these later experiments offer an explanation of the fact that a 

 solution of double the strength produces more tban double the chemical 

 action, they do not explain why it should produce exactly three times the 

 effect, or why the ratio should be the same in all substitutions of this 

 nature hitherto tried. The simplicity and wide range of the 2-3 law seem 

 to indicate that it is a very primary one in chemical dynamics. 



IV. " On the Organization of the Fossil Plants of the Coal-measures.— 

 Part II. Lepidodendra and Sigillarice." By W. C. Williamson, 

 P. U.S., Professor of Natural History in Owens College, Man- 

 chester. Received June 13, 1871. 



(Abstract.) 



The Lepidodendron selaginoides described by Mr. Binney, and still more 

 recently by Mr. Carruthers, is taken as the standard of comparison for 

 numerous other forms. It consists of a central medullary axis composed 

 of a combination of transversely barred vessels with similarly barred cells ; 

 the vessels are arranged without any special linear order. This tissue is 

 closely surrounded by a second and narrower ring, also of barred vessels, but 

 of smaller size, and arranged in vertical laminae which radiate from within 

 outwards. These laminae are separated by short vertical piles of cells, 

 believed to be medullary rays. In the transverse section the intersected 

 mouths of the vessels form radiating lines, and the whole structure is re- 

 garded as an early type of an exogenous cylinder ; it is from this cylinder 

 alone that the vascular bundles going to the leaves are given off. This 

 woody zone is surrounded by a very thick cortical layer, which is parenchy- 

 matous at its inner part, the cells being without definite order ; but exter- 

 nally they become prosenchymatous, and are arranged in radiating lines, 

 which latter tendency is observed to manifest itself whenever the bark-cells 

 assume the prosenchymatous type. Outside the bark is an epidermal layer, 

 separated from the rest of the bark by a thin bast-layer of prosenchyma, 

 the cells of which are developed into a tubular and almost vascular form ; 

 *uit the vessels are never barred, being essentially of the fibrous type. 



