48 A TREATISE ON METAMORPHISM. 



one to the other may be seen by alternately heating and cooling this 

 mineral under the microscope. In the foregoing cases, while we can not 

 doubt that movement occurs, the readjustment is molecular, and it is there- 

 fore beyond the power of the microscope to determine its character. 



It might at first be supposed that such slight movements as are involved 

 in strains within the elastic limit are unimportant, but it is to be remembered 

 that strains of this kind not only affect every mineral particle, but displace 

 the individual molecules with reference to one another, so that the strained 

 masses are affected throughout. While, therefore, it requires polarized 

 light to detect the strained condition in minerals, it is certain that the effect 

 is pervasive. It will be seen (pp. 95-98) that such state of strain is of 

 fundamental importance in the matter of solution and deposition through 

 the agency of solutions. 



In a second class of movements there is molecular interchange between 

 substances by which the compounds are modified in composition. Such 

 interchanges involve chemical action. The motions which occur during 

 chemical changes in solids are commonly for such short distances that the 

 naked eye does not discover the relations of the original and secondary 

 minerals. Such movements are microscopic. Chemical interchange may 

 be mainly accomplished by chemical forces and the movement be an 

 incident of this process. On the other hand, mechanical action may be the 

 inciting cause which leads to chemical action. And, finally, the purely 

 chemical and mechanical forces may interact, each promoting the other. 

 The more important chemical reactions resulting from mechanical action 

 are accomplished through the agency of solutions, and hence are treated 

 in Chapter III. But Prof. Walther Spring" has shown that chemical 

 changes may be induced by mechanical action alone, without the presence 

 of solutions. For instance, when barium carbonate and solid sodium 

 sulphate were mixed in equal molecular proportions and subjected to a 

 pressure of 6,000 atmospheres a change took place by which 80 per cent 

 of the barium carbonate and sodium sulphate were changed to barium 

 sulphate and sodium carbonate, respectively; and conversely, when barium 

 sulphate and sodium carbonate were mixed together in equal molecular 

 proportions and subjected to a like pressure about 20 per cent was changed 



a Professor Spring on the physics and chemistry of solids, review by C. F. Tolman, jr.: Jour. 

 Geol., vol. 6, 1898, p. 323. 



