276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
stead of in octahedrons. The modifications in the crystals of this 
salt produced in a mother liquor which holds methylene blue in solu- 
tion, show that capillarity does not intervene to produce them. In- 
deed, in a solution depositing lead nitrate and saturated with methyl- 
ene blue, without, however, giving crystals of this latter substance, 
the crystals of the nitrate are not at all modified. They are in octa- 
hedrons and colorless, but as soon as the coloring matter begins to 
crystallize synchronously with them the cube faces appear and finally 
are formed to the exclusion of all others. Nevertheless, the surface 
tension can not have been changed since the quantity of methylene 
blue has remained the same in solution. An interesting fact is the 
inequality of absorption of the foreign matter dissolved in the 
mother liquor by the different faces of the crystal. Thus, on the 
octahedral faces of the lead nitrate the methylene blue is not at all 
deposited, but only on the faces of the cube and the pentagonal 
dodecahedron. Similar examples can be cited which explain the 
-appearance, frequent in minerals, known as hour-glass structure. In 
the case of cubic crystals, of all the possible faces it is only those 
which absorb the foreign matter which will develop. 
The idea which first comes to mind is that the molecular structure 
of the crystal plays an important part in this synchronous crystal- 
lization. It is not so at all; different foreign substances may be ab- 
sorbed by different faces, and in such cases the habit of the crystal 
is dependent on these diverse substances. If one causes the colorless 
substance to crystallize in a solution containing two colors, each one 
giving characteristic forms, the crystallizations thus obtained will 
be the two combined forms, so that one and the same crystal is com- 
posed of pyramids or of prisms of different colors. Thus the crys- 
tals of urea nitrate colored by methylene blue and picric acid show, 
if the crystallization has been carefully conducted, yellow triangular 
prisms corresponding to the faces g’ and h’, and blue triangular 
prisms corresponding to the prismatic faces m of the monoclinic 
system. é 
Not only may foreign crystalline matter be absorbed but also the 
liquid matter added to the mother liquid, and even the molecules of 
the latter may also pass regularly into the crystal to modify its form. 
T have been able to show this fact by crystallizing phthalic acid in a 
solution containing ethyl alcohol. This explains why a crystal ob- 
tained from different solvents may show different faces. 
Consequently a crystal, very pure in appearance, transparent, and 
without inclusions, may contain foreign matter, and in the case where 
@To show this, it is enough to take a colored liquid, but with the exception of 
bromine there is no liquid which has a proper color at the ordinary tempera- 
ture. 
