He could not be sure of their support if he declared that the purpose of the war was to free the slaves.

VOA: special.2009.10.08

Generally, there are a couple of elements that one might set down as in a Petri dish, and wait to see if some chemical reaction might take place, but not at all sure of what it might be.

一般来说，都有几个元素,作者会把它们像放入培养基中一样培养,等待某种化学反应的产生，但对可能的结果却一无所知。

普林斯顿公开课 - 人性课程节选

"If you ask them whether they will receive a pension and what age, they say "I'm not sure whether I will be in the position of pensioners of today."

VOA: standard.2010.06.16