Steve wrote:
>  In previous posts I have mentioned that "fog" brings up the notion of concealment

Dear Steve and Others,
I don't often share my thoughts on Eliot.  I am not a scholar or academic.  But I thought I'd give it a try.
Might fog refer to a bewildered state of mind?  Cunfused, muddled?  Or obscured, rather than the more purposeful _concealed_?
Perhaps he is turning his back, ignoring his confusion, keeping it obscure to himself, rather than examining it (looking into the window).

>   The yellow smoke that rubs its_ muzzle_ on the window-panes. ( Muzzle, front of face, nose, etc.)

Might this suggest he had looked into some parts of himself  (though much later he describes himself as "obtuse")
yet other parts he has ignored:
Slipped by the terrace, made a sudden leap,
And seeing that it was a soft October night,
Curled once about the house, and fell asleep.

>    I have often wondered why the "fog" (an obvious image of concealment) in the stanza's opening line becomes the "smoke" in the next line. Since "fog" and "smoke" could almost equally well serve as metaphors for concealment, why is it necessary to have the 'fog' turn into its apparent equal, 'smoke'?

To me, fog settles, while smoke moves.  The fog (obfuscation) rubs its back upon the window (of self examination).  The smoke rubs its muzzle on the window, moves, slips by areas, makes a sudden leap, falls asleep.

Perhaps smoke, rather than the settled fog, is that part of himself which he continues to try to understand and, until he has understood and accepted all of his parts with confidence, that smoke (the unexamined) has the time to slide along the streets, rub its back upon the window-panes, allowing time for him prepare a face.

(concealed desires) and that "yellow" is the color of sickness (perversion).

Yellow is also the color of happiness and sunshine (Light).  In both the fog and smoke, there lingers a hint of light.