That infinite variety of expression
This marvellous invention(language) of composing out of twenty-five or thirty sounds that infinite variety of expression which, whilst having in themselves no likeness to what is in our mind, allow us to disclose to others its whole secret, and to make known to those who cannot penetrate it all that we imagine, and all the various stirrings of our soul.

what is the grammar function of that infinite variety of expression here? as attributive clause? but its structure isn't a sentence, because there's no verb in that sentence.

thank you!

Question

That infinite variety of expression
This marvellous invention(language)  of composing out of twenty-five or thirty sounds that infinite variety of expression which, whilst having in themselves no likeness to what is in our mind, allow us to disclose to others its whole secret, and to make known to those who cannot penetrate it all that we imagine, and all the various stirrings of our soul.

what is the grammar function of that infinite variety of expression here? as attributive clause?  but its structure isn't a sentence, because there's no verb in that sentence.

thank you!

Answer

So, "that infinite variety of expression" : there is no verb, so this is a phrase. It's an expansion of "expression", which is a noun, so you could call it a noun phrase. (The "that" is a demonstrative, the "infinite" an adjective, the "variety" a noun - and they all modify "expression", another noun. ) The grammar of the whole sentence is a little tangled ; is this enough for you?