On the Carboniferous Series of the United States. 407 



manner of change of this substance from its white to its yellow 

 crystalline form. And the following is nearly what happened 

 during its return to its former state. 



When it cools, the white crystals begin to shoot, and if 

 the microscope is adjusted upon one of the yellow hexagons, 

 it is seen to remain quiet and undisturbed until one of the 

 white needles, which elongate rapidly, passes near it. But 

 when the needle passes it, even at what appears in the micro- 

 scope a considerable distance, tlie hexagon becomes corroded 

 on its edges, and then breaks up irregularly, and quickly dis- 

 solves. 



I observed that when a needle, during its growth, happened 

 to strike a hexagon, this seemed to check it for an instant, 

 and then it subdivided itself into a number of ramifications 

 or smaller needles which <li verged from that point; £s if the 

 force (probably of an electrical nature) which caused the 

 growth or formation of the needle-crystal had been deranged 

 or subverted by the disturbing influences which it had met 

 with. 



The change from the white to the yellow form may be re- 

 peated four or five times; but when too much water has been 

 evaporated by the heat, it ceases to occur. The white cry- 

 stals then merely dissolve when heated, without the formation 

 of the yellow ones. 



Remarks. — Are the white and yellow crystals identically the 

 same substance, assuming different forms at different deo-rees 

 of temperature? Is this a case of what has been termed di- 

 morphism ? If I may venture a conjecture, 1 should say that 

 the yellow crjstals are a definite compound of the white cry- 

 stals with water. But however this may be, it appears to me 

 that this and other properties of the iodide of lead are worthy 

 of being more particularly examined.* 



LXXVII. On the Carboniferous Series of the United States 

 of North America. By Richard Cowling Taylor, ^sy., 

 F.G.S., S^-c.\ 



T HAD just completed two articles on the upper series of 

 transiiion rocks, and the relative positions of the deposi- 

 tories of bituminous and anthracitous coals in Pennsylvania, 

 with various detailed illustrative sections, which 1 had pro- 



• These two forms of iodide of lead are noticed in Dr. Inglis's " Ex- 

 tracts from his Prize Etsay on Iodine ;" Lond. and Edinb, Phil. Mag., vol. viii. 

 p. 19. — Eurr. 



t Communicated by the Author. 



