In a post to one of the innumerable technical mailing lists I belong to, a native speaker of German used the phrase lackmus test, meaning a simple method for detecting differences. In English, the phrase is litmus test; why the difference?
Middle English had both the native English word lykemose and the Scandinavian borrowing litemose; only the latter has survived. The second morpheme in each case is that of English moss, but the first morphemes are different, meaning 'drip' and 'dye, color' respectively.
Litmus is made by drying and powdering certain lichens; it was originally used as a water-soluble dye, but is now generally used as a quick-and-dirty test for acidity, hence the metaphorical use of the term (it turns red in acids, blue in bases).
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