Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. A common example is sugar water. If you place a sugar cube in a glass of water, the sugar molecules are concentrated at the sugar cube. Over time, the sugar cube decreases in size. That is because the sugar molecules are dispersing through the water. The sugar molecules are moving from where they are highly concentrated (sugar cube) to where there is a low concentration (no sugar in the water). Eventually, the sugar cube will completely dissolve and the sugar molecules will be evenly dispersed in the container of water. Another example of diffusion is a candle burning. When we light a candle, the room will start to smell like that scent. The ingredients used to make the fragrance are concentrated in the candle; this is where the concentration is high. As the candle heats up, particles that smell like the fragrance are released and dispersed into the air. The fragrance diffuses into the air and now the entire room smells like that scent.
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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