Can anyone shed some light on the interchangeability of these sentences:
I used to think that they are always equivalent and therefore interchangeable but I haven't seen the latter in a while so I'm not sure about its usage. I'd appreciate some examples.
An example of the first could be
I received an email about sproggling today, speaking of which, only yesterday sproggling was mentioned in a lecture.
The second usually has a subject, such as this
I have received a lot of emails about sproggling, mungling and stimming. Speaking of such matters would need some research.
Speaking of which is used by a speaker to refer to a topic that has just been mentioned either by the speaker or someone else.
It is obviously spoken language and comes at the beginning of a new sentence, or could conceivably be placed in a sentence in a dialogue.
Speaking of such can be used in phrase such as; Speaking of such matters can be difficult. But the meaning is not the same as the idiomatic phrase: Speaking of which etc.
Speaking of which is relative, like with which in
I have a wok, with which I cook sometimes.
Nowadays you might be more likely to say “that I cook with”, and analogously “that I'm speaking of” (when I say the following).
I've never used speaking of such, but it seems to me more defensible structurally as the beginning of a sentence. It's the same such that occurs in the phrase as such (when that phrase is not a sloppy substitute for therefore):
I am a member of the club and as such [=as a member] I have access to its facilities.
Speaking of which makes syntactic sense (to me) only as a continuation of the preceding sentence, so it shouldn't be used to begin a sentence in formal writing, in my humble opinion.
"Such" is correct and the same, I just always prefer "which".
