Halliday (2008: 165):
But if we talk of speech as liquid, writing as solid (or whatever metaphorical variant we find suggestive), what particular feature are we referring to? Most obviously, or most immediately, it is the nature of the medium itself; we are looking at speech and writing “from below”. Speech unfolds in time; its material existence takes the form of disturbances in the air; it can be “reduced to writing”, but then it has become writing, like dancers captured in metal or stone. Writing extends in space; it can be “read aloud”, but then it has become speech, like the river flowing out of a frozen glacier. These are just the two modes of material being. But they have different states of meaning, different semiotic modes, associated with them.