differential thermometer A thermometer used for
accurate measurement of very small changes in temper-
ature; usually consists of a U-shaped tube terminating
in two air bulbs and a colored liquid used for indicat-
ing the difference between the temperatures to which
the two bulbs are exposed.
diffraction The scattering of light from a regular
array of points or lines that can produce a constructive
or destructive interference. X-ray diffraction is a tech-
nique used for seeing the structures of crystalline solids.
X rays of a single wavelength are directed at a crystal
to obtain a diffraction pattern in which interatomic
spaces are then determined.
diffusion The random dispersion or spreading out of
molecules from a region of high concentration to one
of low concentration, stopping when the concentration
is equally dispersed.
diffusion-controlled rate See ENCOUNTER-CON-
TROLLED RATE; MICROSCOPIC DIFFUSION CONTROL;
MIXING CONTROL.digestion The process by which living organisms
break down ingested food in the alimentary tract into
more easily absorbed and assimilated products using
enzymes and other chemicals. Digestion can occur in
aerobic conditions, where waste is decomposed by
microbial action in the presence of oxygen, or anaero-
bic conditions, where waste is decomposed under
microbial action in the absence of oxygen. In anaerobic
conditions such as in a large-animal facility (i.e., dairy
farm), the by-product, a biogas, a low-energy gas that
is made with the combination of methane and carbon
dioxide, can be used as an energy source. In analytical
chemistry, the term digestionis used to describe the
process of decomposing materials and bringing them
into solution by heating with a liquid reagent, usually
an acid.digital In signal processing, this refers to the repre-
sentation of quantities in discrete units. The informa-
tion is contained and manipulated as a series of discrete
numbers, as opposed to an analog representation,
where the information is represented as a continuous
signal. In practice, even analog signals are usually pro-
cessed digitally, in that the analog signal is sampled to
create a digital signal that can be processed by inher-
ently digital computers.dihydrofolate An oxidation product of TETRAHY-
DROFOLATEthat appears during DNA synthesis and
other reactions. It must be reduced to tetrahydrofolate
to be of further use.
See alsoFOLATE COENZYMES.dilution The process of reducing the concentration of
a solute in solution.dimer A molecule or compound formed by the com-
bination of two smaller identical molecules.76 differential thermometer
Purple cloud of potassium permanganate diffusing outward as a
solid piece dissolves in a beaker of water, seen from above. The
solid piece (black) takes up much less space than the solution of
potassium permanganate ions. This demonstrates the principle of
diffusion, where a liquid or gas will spread out to fill the avail-
able space. This in turn demonstrates the fundamental differ-
ences between a solid and a gas or liquid. A solid is a rigid,
constrained structure, which contrasts with the fluid, chaotic
nature of a liquid or gas. The molecules of a solid are bonded, but
solvation or melting breaks the bonds between the molecules.
(Courtesy of Andrew Lambert Photography/Science Photo Library)