274 Mr. G. T. Beilby on the Hard 
were not made primarily with the object of ascertaining the 
maximum E.M.F. between the soft and hard varieties, but to 
compare the effects of different amounts and kinds of strain ; 
the soft metal was therefore not necessarily in the condition 
to which it would have been brought by raising it to the 
transition temperature. The highest temperature used in 
Dr. Maclean’s determinations was 100° C. 
Note on the Order of Magnitude of Mobile Films and 
Granules. 
68. This aspect of the molecular changes which take place 
in metal films and on metal surfaces has been pretty fully 
discussed by me on another occasion * ; butas it has a bearing 
on the subject of the present paper, it ought to be referred to 
here. The molecular changes which are due to the formation 
ot the mobile phase occur for the most part in films whose 
thickness is well within the range of the molecular forces. 
When it is remembered that the number of molecules in the 
thickness of a gold-leaf is probably in the region of 900, it is 
plain that ample material for molecular flow is to be found 
in films which are only a small fraction of this thickness. In 
the more easily flowed metals the surface films formed by 
polishing may vary from 1000 to 5000 yp in thickness, while 
in the less easily flowed metals they may be 500 wy or less. 
At surfaces which have moved over each other very slightly, 
films of only a comparatively few molecules in thickness 
might be formed. In calc-spar it was found that a single 
stroke of a leather-covered finger firmly drawn across a fresh 
face was sufficient to break down the crystalline structure for 
a noticeable depth below the surface; the depth of the 
disturbance in this case must, however, have been very 
sheht, 
The granules which are seen in surface-layers have an 
apparent size of from 500 to 2000 wu in diameter; their 
thickness, however, is often very much less than this. 
69. Having regard to the extreme minuteness of the simple 
molecules in metals, it may be safely assumed that relative 
movements of the molecules which are so minute as to be far 
beyond the resolving-power of the microscope, may be sut- 
ficient to undo the work of crystalline aggregation, and to 
transform the C phase into the A. This would hold gocd 
even if the molecules which take part in these changes are 
not simple, but are complexes of relatively large size. 
* Hurter Lecture, Liverpool, 1903; Soc. Chem. Ind. Journ. vol. xxii. 
p- 1169, 
