I have come across "pass off in vapor" or "pass off in fumes" at several places. But I can't seem to find the corresponding definition of "pass off" that would fit in the context.
But neither definition seems to sufficiently explain the following sentences, where the phrase seems to carry the meaning of "evaporate". In fact, MW defines words like "evaporate" and "volatilize" as "pass off in vapor". The meaning of the phrase is pretty obvious, but I just can't find the definition.
Example sentences (from Google books):
The latter plate, being perforated, permits the oil to thence pass onto the next lower ball,and so on downward until it is volatilized and passed off in vapor.
Hydrogen unites with more oxygen to form water, which is passed off in vapor from the lungs, in perspiration from the skin, and in urine from the kidneys.