Answer:
I pass before the sun, but make no shadow.
What has a mouth but can't chew?
We are all around, yet to us you are half blind. Sunlight makes us invisible, and difficult to find.
What can be big, white, dirty and wicked?
My second is performed by my first, and it is thought a thief by the marks of my whole might be caught.
The warmer I am the fresher I am.