emulsion (n.)

"a mixture of liquids insoluble in one another, where one is suspended in the other in the form of minute globules," 1610s, from French émulsion (16c.), from Modern Latin emulsionem (nominative emulsio), noun of action from past participle stem of emulgere "to milk out," from assimilated form of ex "out" (see ex-) + mulgere "to milk" (from PIE root *melg- "to rub off; to milk"). The fat (butter) in milk is the classic example of an emulsion, drops of one liquid dispersed throughout another. Sense in photography is by 1840.