to the air and the mixture contains
hydrogen. The gas is used as a fuel in
some industrial processes.
product See chemical reaction.
progesterone A hormone, pro-
duced primarily by the corpus lu-
teum of the ovary but also by the
placenta, that prepares the inner lin-
ing of the uterus for implantation of
a fertilized egg cell. If implantation
fails, the corpus luteum degenerates
and progesterone production ceases
accordingly. If implantation occurs,
the corpus luteum continues to se-
crete progesterone, under the in-
Ûuence of luteinizing hormone and
prolactin, for several months of preg-
nancy, by which time the placenta
has taken over this function. During
pregnancy, progesterone maintains
the constitution of the uterus and
prevents further release of eggs from
the ovary. Small amounts of proges-
terone are produced by the testes. See
also progestogen.
progestogen One of a group of
naturally occurring or synthetic hor-
mones that maintain the normal
course of pregnancy. The best known
is *progesterone. In high doses
progestogens inhibit secretion of
luteinizing hormone, thereby pre-
venting ovulation, and alter the con-
sistency of mucus in the vagina so
that conception tends not to occur.
They are therefore used as major
constituents of oral contraceptives.
projection formula A conven-
tion for representing the three-
dimensional structure of a molecule
in two dimensions. See fischer pro-
jection; newman projection;
sawhorse projection.
prolactin (lactogenic hormone; lu-
teotrophic hormone; luteotrophin) A
hormone produced by the anterior
pituitary gland. In mammals it stimu-
lates the mammary glands to pro-
duce milk and the corpus luteum of
the ovary to secrete the hormone
*progesterone.proline See amino acid.promethium Symbol Pm. A soft sil-
very metallic element belonging to
the *lanthanoids; a.n. 61; r.a.m. 145;
r.d. 7.26 (20°C); m.p. 1080°C; b.p.
2460 °C. The only naturally occurring
isotope, promethium–147, has a half-
life of only 2.52 years. Eighteen other
radioisotopes have been produced,
but they have very short half-lives.
The only known source of the el-
ement is nuclear-waste material.
Promethium–147 is of interest as a
beta-decay power source but the
promethium–146 and –148, which
emit penetrating gamma radiation,
mustÜrst be removed. It was discov-
ered by J. A. Marinsky, L. E. Glen-
denin, and C. D. Coryell in 1947.
A- Information from the WebElements site
promoter A substance added to a
catalyst to increase its activity.proof A measure of the amount of
alcohol (ethanol) in drinks. Proof
spiritcontains 49.28% ethanol by
weight (about 57% by volume). De-
grees of proof express the percentage
of proof spirit present, so 70°proof
spirit contains 0.7 ×57% alcohol.1,2-propadiene (allene)A colour-
less gas, CH 2 CCH 2 ; r.d. 1.79; m.p.
–136°C; b.p. –34.5°C. Propadiene may
be prepared from 1,3-dibromo-
propane (CH 2 BrCHCH 2 Br) by the ac-
tion of zinc dust. See also allenes.propanal (propionaldehyde) A
colourless liquid *aldehyde,
C 2 H 5 CHO; m.p. –81°C; b.p. 48.8°C.
propane A colourless gaseous hy-
drocarbon, C 3 H 8 ; m.p. –190°C; b.p.
–42°C. It is the third member of the
*alkane series and is obtained from439 propane
p