40 SECTIONAL ADDRESSES. 
the substance used being sublimedina vacuum. This has been undertaken 
by Volmer, who finds that cadmium, zinc and mercury crystals grow in 
this way in a high vacuum. When small nuclei are present, those grow 
which have the face with the smallest velocity of growth perpendicular 
to the stream of impinging molecules. The differences between different 
faces are large, so that under these conditions either flat tables or long 
prisms are usually formed, according to the direction of the original 
nucleus. The crystal grows by the addition of thin laminz, probably 
only one molecule thick, which spread over the surface. This is likely 
to be the process when the crystal is growing in a solution or in a molten 
mass, as well asin the vapour ; and, in fact, when cadmium or tin is being 
deposited electrolytically at a cathode, or when lead iodide is being formed 
from a solution of a lead salt and an iodide, the growth of the crystal may 
be watched under the microscope, when a thin film begins to form at some 
point on a face, and extends over the face, maintaining a uniform thickness 
throughout. Marcellin had previously observed the same thing in 
p-toluidine, the layers not being more than two molecules thick, and 
probably only one. Marcellin also found that mica might be cleaved by 
Wood’s method of pressure against fused selenium, until the lamine 
had a thickness of one molecule. Moreover, there are indications that 
when molecules strike the surface of such a fresh crystal they first attach 
themselves irregularly in what is now called an adsorbed layer, before the 
film takes up regular orientation. It is realised that in the presence of 
a foreign substance either molecules or ions may attach themselves to 
such a surface by their residual affinity, and this will necessarily affect the 
addition of further layers of the original substance. In other words, the 
velocity of crystallisation in a direction normal to that face will be changed. 
As the residual affinity of different faces of a crystal must, from the ordinary 
conception of an atomic space lattice, be different, the habit of the crystal, 
that is the relative development of different faces, will be altered by the 
presence of a foreign substance. There is, in fact, evidence that dyes are 
not equally adsorbed by different faces of the same crystal, so that the 
state of things just imagined must exist. It is on these lines that an 
explanation of differences of habit must be sought. 
This possible effect of very minute quantities of impurities reminds us 
that we know exceedingly little of the properties of pure solids. Gases 
and liquids, which we commonly assume to be easily obtained in a pure 
state, have been shown, especially by Baker, to alter greatly in properties 
when deprived of their last traces of moisture, and this is true to some 
extent even of solids, Baker having found that specimens of sulphur and 
iodine had their melting points raised by 5.5° and 2° respectively when 
submitted to intensive drying for nine years. Another illustration may 
be taken from the effect of dissolved gases on metals. Most metals as 
cast contain very considerable quantities of gases, either in true solution 
or trapped during freezing by the growth of neighbouring crystals, and 
these gases are not removed completely in the later operations of forging 
or rolling. The effect of gases on the physical properties of the metal has 
been little studied, but that it may be great is shown by the instance of 
soft iron used for transformer cores. Hither commercially pure iron or 
the alloy of iron with silicon which is commonly used for this purpose is 
enormously improved in its magnetic properties by melting in a high 
