A mutagen is a chemical or physical substance that corrupts genetic material, such as DNA or RNA. Mutagens create genetic mutations and increase an organism's risk for cancers and cell death. The two main types of mutagens are physical and chemical. A physical mutagen is in the form of a physical substance. The main physical mutagen is short wave rays which penetrate living cells and alter genetic material. Examples of this type of mutagen include ultraviolet rays and X-rays. A chemical mutagen is a mutating agent in the form of a chemical substance. These substances work by mimicking nitrogenous bases in genetic material that cannot be duplicated during DNA replication. Examples of this type of mutagen would be nitric acid or peroxide.
Does anybody know the etymology of the phrase "sweet dreams"? I tried googling but did not find anything satisfying. Is this a relatively new phrase of the modern world or has this been in use for some time? I think it's the latter one. Answer The OED has the interjection as "a farewell to someone going to bed" from the 20th century: 1908 Sears Roebuck Catal. 198/1 Tenor Solos..Good Bye, Sweet Dreams, Good Bye. But it goes back until at least the 19th and possibly 18th centuries. John Wolcot, writing under the pseudonym of Peter Pindar, used it in his poem "Orson and Ellen; A Legendary Tale" published in 1801: Also from 1801 in The infernal Quixote (Page 287) by Charles Lucas: In the March 1776 of The Universal Magazine was published "The Serenade. A Pastoral Tale. From the German of Gesner" , where the shepherd Daphnis watches over his beloved as she sleeps and sings: The same tale appears in 1776's as Idyl XI, " Daphnis ...
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