The mist in Porius flows up to a man and he dies, not for any of the sane reasons that mists in fiction kill people (then it would be constructed out of poison) but because the two of them are ideologically opposed. Slightly earlier in the book this same man gave a speech and the sound of his voice went out into the forest until it came to a clearing where a corpse was lying on the earth, and at that corpse it stopped. "The voice was lost in fact with that sort of abrupt shock that we experience when we unexpectedly touch the ice that is called cat-ice, from beneath which all the water has been drawn away."
(It touches then, this voice. Where are its fingers? Where are its nerves? Lovely beyond all things, that there is no answer to that question: it has them.)
The world that Powys likes to describe is limitlessly anthropomorphic and casually miraculous. The miracles are not the property or responsibility of a god, they are the native behaviour of the world, and it is natural for everything to have a personality, and for the personality of a voice to not be able to bear the personality of a corpse. For every atom is enpersoned.
They have eccentric likes and dislikes, as did Powys himself, according to his Autobiography. He could not stand the thought of dogs being vivisected, or of a person looking at their own snot in a handkerchief. I believe that vivisection was on his mind when one of the Ancient Britons of Porius puts a spear through a dog.
Where does an author stop, when he could have made the mist talk if he'd wanted to, or had it do anything else; make it coalesce into the form of Shane Warne who will bow and recite his own Twitter feed, "Still feeling good & happy with Smiths decision to bowl first ? That's right, you guys weren't worried ! Go on Mitch"? Why shouldn't the mist do that?
Here is my thought. I am sentimental. The author tip-toes out and stops or starts with their own nature, smelling the scent of that and writing or not writing, this nature being inconstant as you can see in Porius, not always carrying a thought on but sometimes carrying it on, not always creating a murdering mist but once upon a time creating one, not always, but sometimes, explaining the personality of a bee, and on one page, and one page only, putting "the whole chemical substratum of planetary elements" inside the word bubble.
[S]he paused and turned and stared down in fascinated wonder while a particular sequence of tiny bubbles met and lost themselves in a larger one, and indeed kept on increasing the size of this larger one till it came to resemble a sub-human and at the same time a super-human eye! It became in fact the living eye of the whole chemical substratum of planetary elements, out of which water, earth and air draw the fluidity, the solidity, the imponderable vacuity of their essential beings. The girl bent down as she crossed this little stream and stared steadily into one of these constantly-growing bubbles; and this arbitrarily chosen eye of the whole inorganic world stared back at her -- the eye of matter itself responding in sub-conscious or perhaps super-conscious indifference to the eye in a human skull!
The large mass of the fluctuating human attention-focus is mapped out in Porius, like coral.