PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 765 



SO rendered more visible, by the contact of rubber with water, 

 alcohol, or fused sulphur. They are most readily seen in a film 

 produced by the evaporation from a solution of rubber and bi- 

 sulphide of carbon. It is these pores Avhich give rise to fibers 

 when the rubber is forcibly stretched. Paycn contends that water 

 and other liquids are carried into these pores by capillary attrac- 

 tion, and are not diffused through the whole substance as is the 

 case when gelatine or other true colloids absorb liquids. He con- 

 cludes, therefore, that gases pass through rubber septa, just a3 

 they do through septa of graphite, or of unglazed earthenware. 



Petrifaction. 



True petrifaction is a process of fossilization in which the oi"i- 

 ginal mold or cast of an organic fossil is filled up with a kind of 

 matter differing from the original body. Deposits on the surface 

 of objects are sometimes met with which are commonly called 

 petrifactions. A sample of this is the petrified moss at some of 

 the mineral springs in western New York. The carbonate of lime 

 forms aM incrustation around the plant. Another variety is sili- 

 cious deposits on moss, ferns, and other objects found near the 

 hot springs of Iceland. Partial fossilization sometimes occurs, as 

 in the case of the common clam, for instance, which is often found 

 filled with mineral matter while the shell remains intact. A true 

 petrifaction is where the inorganic structure of the whole animal 

 or plant is replaced by mineral matter. Examples of this are 

 petrified palm-trees which are entirely changed in their composi- 

 tion, and yet retain all the fibers and cells in their original form 

 and minuteness, so that, when viewed by the microscope, the spe- 

 cies of the tree can be determined. Another common case in point 

 is the echinus, in chalk formations; its shell has been changed to 

 calcspar, while its interior has been filled with fiiut. The testa- 

 ceous animal has the power of changing the crystals of carbonate 

 of lime in the construction of its shell, but when life has departed 

 the laws of crystalization resume their sway and the carbonate as- 

 sumes its original rhomboidal form. 



In the beautiful little shell, shaped like a ram's horn, called the 

 ammonite, iron pyrites is sometimes found. It is supposed the 

 sulphur in the animal has united with iron, and although the natu- 

 ral form of the bisulphide of iron is cubical, in this substitution 

 process it is forced to take the rhomboidal shape. In fossilization 

 or petrifaction the organic matter may be replaced by various 



