The Promise of Air by Algernon Blackwood

He plunged into the stream of pedestrians and it struck him how thickly, heavily clothed they were; the street resembled a sluggish river of dark liquid; he struggled through it, immersed to his shoulders.

And the flock of curious, elusive thoughts, half-formed, fluttered above his mind just near enough to drop their shadows before they scattered and passed on. Much as a kitten pounces on the shadow of shifting foliage on a lawn his brain pursued and pounced upon them, bringing up the best words available, yet that did not suit because the necessary words do not exist. It was only the shadow of the ideas he captured.