476 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9. 
present-day concept of a crystal as an arrangement of atoms so placed that 
all of them lie in planes which are regularly spaced. The cubical crystal 
of common table salt will serve as an illustration. One could think of the 
structure of such a crystal as somewhat like that of a cubical box exactly 
filled with equal-sized oranges, each orange representing an atom. The 
oranges would arrange themselves in layers, and no matter whether the 
box were standing on a flat side or balanced on an edge or on a corner, there 
would be, horizontally, layer above layer. When the box is standing with 
a flat side on the floor, the distance between the center planes of the layers 
will be, say, i unit. If the box is balanced on an edge, the distance between 
the new horizontal planes will be less than the unit; actually it will be 
J V2 (or .707) times the unit. If the box is balanced on a corner, then 
the planes which are now horizontal will be still closer together, actually 
J V3 (or .577) times the unit. In each case the distance between the atoms 
of a single plane becomes greater. That is, as the planes become closer 
together the atoms in the planes become farther apart. There are many 
other planes, but there is no need of further discussion of them here. Of 
course, the atoms may be relatively farther apart than the oranges in the 
illustration, leaving considerable space between them. One can visualize 
this arrangement as a sort of lattice with the atoms at the corners, a three- 
dimensional lattice. The expression "space lattice" is commonly used in 
this kind of work. 
In mineralogy there are several ways of designating the planes described 
above ; the system generally used in the X-ray work with crystals designates 
the first plane as the 100 plane, the second as no, and the third as in. 
These figures refer to a relation between the planes and the axes of the 
crystal. More detail concerning the crystal is probably not necessary here. 
X-Rays 
The modern conception of the atom is that it consists of a core, or 
nucleus, surrounded by a system of electrons, which are situated at a 
considerable distance, relatively, from the core. The electrons may be 
arranged in concentric rings, the number of both electrons and rings varying 
with the different elements. One or perhaps several of these may be 
shot off from the atom by appropriate means without changing the ele- 
mental nature of the atom. 
If a high-voltage electric current is allowed to pass through a vacuum 
tube which has at its cathode a fine wire spiral at white heat, a stream of 
electrons will be shot off from the spiral to the anticathode. They will 
attain a velocity somewhat less than that of light. If this stream of elec- 
trons is allowed to impinge on a metal anticathode, the atoms of that metal 
will be jarred into vibration as the electrons hit them. This vibration 
starts a series of waves in the surrounding ether, from each atom. These 
waves are X-ray waves. The wave-lengths are of the order of the Angstrom 
