Think of a jar of oil and vinegar standing in two clean layers, and then someone shaking it hard. The oil does not vanish. It is cut into drops too small to see, and those drops hang all through the vinegar, which is why the liquid turns cloudy and thick instead of clear. That is the picture the whole word is built on: fine particles of one thing spread evenly through another and held there. Milk is that. is that. So is the pale, creamy paint in a tin, and so is the thin layer on a strip of photographic film — in every case, something has been broken up small and spread out, and the mixture stays as one until something makes it separate again.
From the word’s history
The supplied source says emulsion was borrowed from French émulsion, from New Latin ēmulsiō, based on Latin ēmulgeō, to milk out or extract. Milk is the oldest example of the thing itself: fat squeezed out into tiny drops and spread through water, which is why it looks white rather than clear.
本次證據裡的 Wiktionary 記載：emulsion 借自法文 émulsion，源頭是新拉丁文 ēmulsiō，而它建立在拉丁文 ēmulgeō（擠出、擠奶）之上。可以想成最原始的例子就是牛奶本身：脂肪被擠成極小的油滴，散在水裡，所以看起來是白的而不是透明的。今天的每個用法，都還帶著那杯牛奶的樣子。
Wiktionary entries supplied in this packet (source_refs kaikki:3f6f69a190036ae4:1002231712:10345 and :1002242057:6303)
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