i‘ B.—CHEMISTRY. 39 
The great beauty of natural crystals has attracted the attention of poets 
and artists as well as men of science. Much of this beauty depends on 
the varying habit of one and the same crystal species. Even with such a 
common mineral as quartz, it is possible on entering a mineral collection 
to point to some of the crystals exposed, and to name their locality, when 
once the form has become familiar, The same is true of other minerals. 
Why should there be this variation, when the chemical composition of the 
distinct varieties may be identical, so far as analysis is able to give 
- information? Again, the crystalline system will not account for the 
differences in the building up of individuals to form aggregates. Rock 
salt and cuprite crystallise in cubes, and the space lattice has a very 
similar form in the two minerals, but when the salt forms multiple growths, 
the cubes arrange themselves in characteristic stepped pyramids, whilst 
_ the red oxide of copper may form the most beautiful hair-like threads, a 
tissue of scarlet silk, as Ruskin calls it. Neither mineral ever assumes 
a form which is characteristic of the other, the simple cube being once 
departed from. Why should this be? It is known that the presence of 
traces of foreign matter may cause differences of habit, the most famous 
instance being that of the crystallisation of common salt in octahedra 
instead of cubes when 4 small quantity of urea is added to the solution, 
but the explanation of these facts is still imperfect. 
An important paper on this subject was published in the Annales des 
Mines as far back as 1818, by F. 5S. Beudant, who examined a large number 
of minerals and salts with the object of discovering the causes of variations 
of habit, concluding that the most important factor was the presence of 
foreign substances. This paper probably contains a larger mass of data 
than any later publication on the subject. Among recent workers, 
Gaubert has made an interesting study of the influence of impurities, 
especially of colouring matters, on habit. It was a problem which 
fascinated Ruskin, whose intimate knowledge of the forms of minerals, 
and keen desire to understand the reasons for their varying beauty, 
combined with a penetrating insight into natural phenomena, might have 
led him to discoveries of importance had he received greater help from his 
scientific friends. As it was, his chief contribution to the subject was his 
series of studies on agates and other banded formations, in which he 
anticipated some of the conclusions lately reached by Liesegang by entirely 
different methods, showing that the bands were produced by segregation 
from a gelatinous mass, and not, as had been supposed, and maintained 
until a few years ago, by the successive infiltration of fresh quantities of 
solution into a cavity. 
According to Curie, the appearance of a given face on a growing crystal 
depends on the ratio of its surface energy to that of other possible faces, 
but it has been found that such differences of surface energy as occur are 
much too small to account for the effect. The work of Johnsen and of 
Gross has shown that the appearance of a face on a crystal placed in a 
supersaturated solution is really determined by the velocity of growth in 
a direction normal to that face, those faces being produced which have a 
minimum velocity of growth. The presence of impurities undoubtedly 
has an influence on the velocity, although the effect of very small quantities 
of impurity has been little studied. Some light is thrown on the subject 
_ by a study of the growth of a crystal when solvent is completely excluded, 
