56 A TREATISE ON METAMORPHISM. 



When the modified magma crystallizes it takes the form of an ordinary 

 igneous rock, and may show no evidence of the fact that previously 

 solidified rocks have contributed material. 



chemical effects. — In proportion as the temperature is high chemical reactions 

 are likely to take place between solids. This is illustrated by the case- 

 hardening of iron. When soft iron is placed in contact with pulverized 

 charcoal and the temperature is raised to a red heat, but not to the point 

 of fusion, some of the carbon unites with the iron, transforming the outer 

 part of it into steel. Thus it is casehardened. Just how the union takes 

 place between the iron and the carbon is uncertain. It is supposed to be due to 

 the direct union of the solids, but we can not be quite sure that the result 

 is not accomplished through the agency of a gas. The carbon may be partly 

 oxidized, and thus be transformed to the gas carbon monoxide. This may 

 penetrate the iron, which may reduce the carbon monoxide to carbon again. 

 The reduced carbon may at the instant of reduction unite with the iron, 

 forming' the carbide, or steel. While it is certain that high temperature is 

 favorable to the mutual chemical reactions of solids, when the temperature 

 becomes so high as to transform the solids to liquids the chemical reactions 

 are those of liquids rather than those of solids. 



INDIRECT EFFECTS OF HEAT AND LIGHT. 



The indirect effects of heat and light are accomplished through the 

 agents of metamorphism — gases, water, and organic forms. The move- 

 ments of the atmosphere and hydrosphere are the conjoint effect of heat 

 and light and gravity. It has already been noted that the movements of 

 these bodies are the agents which do the main work of epigene transfer 

 of material. Not only do gas and water act as agents of transfer, but they 

 act as agents for chemical changes. It has already been seen that chem- 

 ical action may be a direct result of heat. However this may be, it is 

 certain that by far the more important, indeed the dominant, effects which 

 heat and light have upon chemical reactions are accomplished through the 

 agency of gases and water and organic forms. Of the forces heat and 

 light, the former is the important one in the reactions accomplished through 

 the agency of gases and water solutions ; but light is very important in the 

 production of organic agencies. The indirect effects of heat and light and 

 all other conjoint forces are considered in connection with the agents of 

 alteration in Chapter III. 



