A Question of Grammar

In which is it which or is it that?

I floundered with grammar (and often still do) until age 30 when I was in my first and only year of graduate school, studying journalism. One clear-speaking professor and one simple wire-bound book helped me to somewhat demystify the quagmire. Now I return to When Words Collide: A Journalist’s Guide to Grammar and Style whenever a thorny issue of grammatical proportions arises, such as this issue of that vs. which. Is it correct to say:
Beeswax candles emit negative ions that clean the air;
or, should it be:
Beeswax candles emit negative ions which clean the air.
In which case, the latter case, shouldn’t there be a comma after ions? Due to the fact that (and now I am consulting the book) clean the air is a non-restrictive clause elaborating on negative ions? So:
Beeswax candles emit negative ions, which clean the air.
That introduces a restrictive clause, a clause used to restrict meaning, so meaning to say that only some negative ions clean the air. Such as those emitted from a beeswax candle?

One could say, the very crux of the matter.


Reference
When Words Collide: A Journalist’s Guide to Grammar and Style


Page edited 1/16/19

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