Perc is classified as a hazardous air contaminant by the United States Environmental Protection Agency and must be handled as a hazardous waste. To prevent it from getting into drinking water, dry cleaners that use perc must take special precautions against site contamination. Landlords are becoming increasingly reluctant to allow dry cleaners to operate in their buildings. When released into the air, perc can contribute to smog when it reacts with other volatile organic carbon substances. California declared perchloroethylene a toxic chemical in 1991, and its use will become illegal in that state in 2023.
Some alternatives such as CO avoids the toxic risks of perc. But, CO does not clean some forms of grime as well as perc.
Various commercial products on the marketplace today, such as Procter & Gamble's Dryel, allow elements of the dry cleaning process to be performed in the household using home laundry machines. Though not the complete process that would be performed by a professional dry cleaner, they allow the convenience of home laundry and work for certain types of garments.
Evolution of cetaceans
The Cetaceans (whales, dolphins and porpoises) are mammalian marine descendants of land mammals. Their terrestrial origins are indicated by:
The question of how land animals evolved into ocean-going leviathans was a mystery until recent discoveries in Pakistan revealed several stages in the transition of cetaceans from land to sea.
After many years when palaeontologists thought whales had evolved from mesonychids, DNA sequence analysis showed the closest relationship was with artiodactyls, the even-toed ungulates. A new clade was created to include Cetacea and their nearest relatives, the hippopotamus family. This clade is called the Cetartiodactyla.
Hippo fossils are not found until the Miocene, but whale ancestors have been found from the Eocene. This leaves a gap of nearly 30 million years where no hippo ancestors are found. The most recent hypothesis is that hippos and whales shared a common semi-aquatic ancestor which branched off from other Artiodactyls around 60 million years ago (mya). This ancestral group probably split into two branches around 54 mya. One branch evolved into cetaceans, possibly beginning with the proto-whale "Pakicetus" from 52 mya. These early cetaceans gradually became adapted to life in the sea. They became the completely aquatic cetaceans.
"Indohyus" is a small deer-like creature, which lived about 48 million years ago in Kashmir. It belongs to the artiodactyls family "Raoellidae", and is believed to be the closest sister group of Cetacea.
About the size of a raccoon or domestic cat, this herbivorous creature shared some of the traits of whales. It also showed signs of adaptations to aquatic life, including a thick and heavy outer bone coating. This is similar to the bones of modern creatures such as the hippopotamus, and reduces buoyancy so that they can stay underwater. This suggests a similar survival strategy to the African mousedeer or water chevrotain which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.
Scientists from the University of Plymouth studied many animals that dive and hold their breath, from insects to whales, and they found that larger animals can hold their breath longer than smaller animals because they can store more oxygen for their size, and this difference was much bigger for warm-blooded animals than for cold-blooded animals. They said this may be why modern whales and extinct diving animals like plesiosaurs became so large.
Coal ball
A Coal ball is a permineralised life form that is full of calcium, magnesium and occasionally iron sulfide. They generally have a round shape. Coal balls are not made of coal, even though they have the name "coal ball".
In 1855, two English scientists, Joseph Dalton Hooker and Edward William Binney, found coal balls in England. Because of that, the initial research on coal balls was done in Europe. Coal balls were found and identified in North America in 1922. Since then, coal balls have been found in other countries and several theories on their formation have been proposed.
Marie Stopes and D.M.S. Watson also examined coal ball samples. They agreed that coal balls formed "in situ". They also added that interaction with a marine environment was necessary for a coal ball to form.
Coal balls are in coal seams across North America and Eurasia. North American coal balls are in more places than in Europe. The oldest coal balls were found in Germany and former Czechoslovakia.
In 1962, Sergius Mamay and Ellis Yochelson found signs of marine animal remains in North American coal balls.
The quality of the preservation of organic material depends on the speed of the burial process and the degree of compression before undergoing permineralisation. Generally, coal balls resulting from remains that have a quick burial with little decay and pressure have a higher preservation degree. However, plant remains in most coal balls show various signs of decay and collapse.
Thin sectioning was an early procedure used to analyse fossilised material contained in coal balls. The ball was cut into thin sections with a diamond saw. Then it was flattened and polished with an abrasive. Then it was examined under a microscope. This is the procedure which was carried out by Hooker and Binney. The time needed, and the poor quality of samples produced, led to a more convenient method.
The new method, first used in 1928, is called the "liquid peel technique".
Napster
Napster was a computer program that allowed people to share music over the Internet. It was shut down as the result of a lawsuit by the RIAA. Napster has since reopened with some changes
Paul Flory
Paul John Flory (June 19, 1910 – September 9, 1985) was an American chemist and Nobel laureate. He worked in the field of polymers, or macromolecules. He was a leading pioneer in understanding how polymers dissolve in solutions. He won the Nobel Prize in Chemistry in 1974 "for his fundamental achievements, both theoretical and experimental, in the physical chemistry of macromolecules."
He graduated from Elgin High School in Elgin, Illinois in 1927. Flory received a bachelor's degree from Manchester College (Indiana) in 1931 and a Ph.D. from the Ohio State University in 1934. His first position was at DuPont with Wallace Carothers.
Flory's earliest work in polymer science was in the area of polymerization kinetics at the DuPont Experimental station. Most chemists studying condensation polymerization believed that the reactivity of the end group decreased as the macromolecule grew. Flory argued that the reactivity was independent of the polymer's size. He showed that the number of polymer chains present decreased with size exponentially.
Flory introduced the important concept of chain transfer to the study of addition polymerization. This improved chemists' understanding of the kinetic equations. It also helped chemists to understand the distribution of the polymer sizes.
In 1938, after Carothers' death, Flory moved to the Basic Science Research Laboratory at the University of Cincinnati. There he developed a mathematical theory for the polymerization of compounds with more than two functional groups. He also developed the theory of polymer networks or gels.
In 1940, he joined the Linden, NJ laboratory of the Standard Oil Development Company. He developed a statistical mechanical theory for polymer mixtures there.