B.—CHEMISTRY. 538 
or sputtering in a vacuum are probably the most under control, but other 
surfaces are commonly covered by invisible films. Little trust is to be 
put in determinations of the angle of contact of liquids with solids, a 
property of great theoretical and practical importance, since the solid 
surface actually examined is covered with a film of foreign atoms. 
Schumacher has recently shown that mercury wets glass and silica more 
and more readily as care is taken to remove films from them, and the 
property of not being wetted by mercury is probably not one of glass 
and silica, but of those substances coated with a film of gas. Metals 
most readily take up atoms of oxygen or other elements, forming persistent 
films, which play an important part in the phenomena of resistance to 
corrosion. Purely physical theories of passivity are not satisfactory, and 
it seems to be impossible to explain that property without assuming the 
presence on the surface of an invisible film, which is probably responsible 
for, among other things, the high resistance of certain chromium steels 
and other alloys to corrosion. 
There is one way of preparing a fresh surface of a crystalline solid 
for examination, and that is by cleavage. A freshly cleaved plate of a 
mineral may be supposed to be clean at the moment of its formation, 
although it will rapidly take up foreign atoms from the surrounding gas. 
It was known as far back as 1846 that a fresh cleavage of mica had different 
properties from one which had been exposed to the air for a time, and 
this was attributed by P. Reiss to the absorption of moisture. Tammann 
has made the interesting observation that a fresh surface of mica is more 
soluble in water than an older one. Washing with water immediately 
after cleaving extracts a quantity of alkali salts which is much above 
the normal solubility of mica, and later washings extract only the normal 
quantity. Itis suggested that the separation of the flakes of mica exposes 
the alkaline part of the molecules, which would be more readily attacked 
by water than the silicious part. Assuming that molecules are arranged 
perpendicularly to the cleavage planes, we may think of the act of cleaving 
as exposing the soluble ends, as if the molecules of mica were an array of 
hermit crabs with their soft unprotected ends exposed to attack. It will 
be interesting to see whether the X-ray examination of mica confirms this 
arrangement. Again, however, a word of warning as to the effect of 
possible impurities must be uttered. Natural minerals are not pure, and 
any uncombined alkaline salts present might well segregate along cleavage 
planes in the process of crystallisation, and so give rise to the effect noticed 
above, but the figures recorded by Tammann are striking and suggestive. 
In this hurried review of a large field it may seem that I have presented 
rather our ignorance than our knowledge, my intention having been to 
show how much remains to be done before we can understand the chemical 
relations of solids as we do those of liquids and gases. One department 
of research is, however, more advanced than might have been supposed 
from my brief references to it. That is the study of the internal changes 
in metallic alloys as revealed by the microscope and by thermal and 
electrical methods. Metallography has made wonderful progress since 
the days of Sorby, and it would repay students of physical chemistry to 
_ give some attention to its main results, even though they may not intend 
to make a special study of the subject. Nowhere are the benefits of the 
doctrine of phases of Willard Gibbs to be more clearly traced, whilst the 
