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Moment of truth
The Voyager I spacecraft is the most distant man-made object in the universe. It was launched in 1977 and as in 2002, it was more than 8 billions miles from the Earth.

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peacock ore Min.
See bornite.

penicillin Biol., Biochem.
A natural antibiotic obtained from culture filtrates of Penicillium notatum, a fungal mould. Penicillin is not a single compound, but a group of closely-related compounds, derived from two amino acids cysteine and valine. It kills newly form bacteria by disrupting the synthesis of the bacteria cell wall. Hence it is effect only on bacteria (of Gram positive type) with certain cell walls such as Staphylococcus aureus. The antibiotic effect was accidentally discovered by Alexander Fleming in September 1928. However, large-scale production of penicillin became viable only by Howard Florey and Ernst Chain, two British scientists working in the United States during Second World War. All three scientists were awarded the Nobel Prize for medicine in 1945.

pepsin Biochem., Biol.
A protease enzyme produce by cells lining in the walls of the stomach of mammals and human. It catalyses the breakdown of protein into shorter peptide chains (proteolytic process). It operates in the acidic environment of pH 1.5 to 2 (in stomach) but become inactive at neutral pH (when entering intestine).

peptide Biochem.
A sequence of amino acids joined together by peptide bonds. The sequence can consist of anything from two (dipeptide) to several hundred (polypeptide) amino acids. A biological molecule such as protein consists of peptides folded in certain three-dimensional structure to give its biological function.

peptide bond Biochem.
Chemical structure -CO-NH- acts as a link between two amino acids. It is formed via condensation reaction (removal of a water molecule) between the a-amino group from an amino acid and the carboxyl group of another amino acid. The building blocks of a protein consists of a linear array of amino acids called peptide and the individual amino acid components are joined together by a series of peptide bonds.

percolation theory Phys.
A computational technique to model complex many-body interacting system by simplifying the interactions between the individual constituents to some lowest form from the reality and allowing the system to evolve in computer simulation. Generally, the idea is to study the overall behavior (macroscopic) of the system model as a result of the behavior of individual constituents. Example study is the simulation of disease propagation in a population of living organisms. The technique is also developed to deal mathematically with disordered and porous media whereby the disorder is defined by a random variation in degree of connectivity.

perihelion Astron.
The point at which an object, such as a planet or comet, orbiting the Sun which is nearest to the Sun.

permanent gas Chem.
A gas that cannot be liquified by compression alone at normal temperature. For example, carbon dioxide cannot be liquified when pressure is applied at room temperature, until the temperature is reduced to a point (the critical temperature) where the gas can now be liquified. The term was first coined by Michael Faraday (1791-1867) when he failed to liquify hydrogen and oxygen even up to few thousand times the atmospheric pressure.

permanent hardness Chem.
The hardness of water which cannot be removed by boiling. This is due to the presence of soluble calcium and magnesium chlorides or sulfates which do not alter chemically by boiling, Such waters usually need chemical treatment, such as addition of sodium carbonate, in order to remove the hardness.

peroxisomes Biol.
Organelles in eukaryotic cell that contain enzymes such as catalase, urate oxidase and D-amino acid oxidase. Peroxisomes are surrounded by a single lipid bilayer membrane that separates the contents from cytosol (cell fluids). They are important in the elimination of toxic peroxide anions by fatty acid oxidation and free radicals.

petrochemicals Chem.
Organic chemicals that are obtain from petroluem and, to a smaller extent, natural gas. Most of these chemicals are used as starting chemical feedstocks for the industrial productions of other chemical compounds. Petrochemcials are also used in making polymers, solvents, adhesives, resins, lubricant, etc. Some of the most common and important petrochemicals are ethene, propene and benzene.

petroleum ether Chem.
A colorless, volatile liquid hydrocarbons containing mainly a mixture of pentane and hexane but may contain a small amount of branched alkanes such as methylpentane and dimethylbutane. It is obtained from petroleum refinaries that distilled between 30C and 70C. It is extremely flammable, more so than gasoline and is used as a non-polar solvent. However, petroleum ether is not a member of ether organic compounds which contained the ether oxygen (-O-) functional group. Petroleum ether is also commonly known in several other names such as benzine, ligroin, petroleum spirits etc.

pewter Eng.
pewter vase A silvery alloy of at least 63% tin. It is used to make tablewares and other ornaments (such as vase, as shown on the right). Nowadays, pewter products are mostly made of 97% tin with other balance metals such as copper, bismuth, silver and antimony. Unlike silver, pewter is resistant to tarnish and also corrosion. For this reason, it is regarded as a precious metal, after platinum, gold and silver.

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