462 Notices of Memoirs — 



endowed with other orderly movement that may be likened to the 

 motion of the planets round the sun ? Far, far away in space 

 the solar system would, to an eye formed like our own, in all 

 probability present a nebulous appearance, because the eye would 

 not be able to see the individual members of our system. So, too, 

 the molecules of which crystals are built up may have their appro- 

 priate motions, but we cannot see them with the eyes of sense 

 because the molecules are beyond the highest powers of the 

 microscope. We can, however, I think, perceive them with the 

 eye of the scieutific imagination ; and the hypothesis that the mole- 

 cules of minerals are separated from each other by intermolecular 

 spaces, and have their modes of motion, seems essential to the 

 comprehension of rock metamorphism. 



The important experiments of Sir W. Roberts-Austen on the 

 diffusion of gold in pure lead throw considerable light on this 

 subject. Disks of solid gold were held against the bases of cylinders 

 of lead by clamps, and were kept in an upright position at the 

 ordinary temperature for four years. At the end of this time it 

 was found that the gold had diffused upwards in the solid lead, 

 for a distance of 7*65 mm., in sufficient quantity to be detected by 

 the ordinary methods adopted by assayers. Traces of gold were 

 found still higher. When a column of molten lead, 16 cm. high, 

 was placed above solid gold and kept at a mean temperature of 

 492° C, that is to say, at 166° above the melting-point of lead, 

 but 569-7° below that of gold, the gold diffused in considerable 

 amount, to the top of the lead column, in a single day. 



Sir W. Roberts- Austen's experiments, above alluded to, demonstrate 

 that even such metals as gold, whose melting-point is as high as 

 1061"7° C., exhibit a measurable amount of kinetic energy at the 

 ordinary temperature and pressure. Great results may no doubt 

 be brought about at ordinary temperatures and pressures, when 

 time, as in the laboratory of nature, is practically unlimited ; never- 

 theless, the importance of high temperature and high pressure, in 

 operations connected with metamorphism, can hardly be overrated. 

 Not only does a rise in temperature increase the energy of the 

 chemical actions and reactions which produce the mineralogical 

 changes embraced by the term metamorphism, but it increases the 

 porosity of minerals and facilitates the passage of liquids and gases 

 through their pores. The cohesion of molecules is lessened, the 

 amplitudes of their vibrations, rotatory or other movements, are 

 increased, and a passage is opened for the advance of chemical 

 materials into the heart of the crystal. 



Increase of temperature thus not only throws open the doors of 

 the mineral fortress attacked, but gives enhanced energy to the 

 invaders. The fact that the mineral components of a rock are, 

 under conditions of heat and pressure practically porous to heated 

 Avater, laden with chemical reagents in solution, is frequently 

 brought home to the mind of the petrologist in a very tangible way. 

 We sometimes observe, for instance, that metamorphic changes 

 begin at the heart of a crystal, and leave the peripheral portions 



