Riddle

From rosy bowers we issue forth,
From east to west, from south to north,
Unseen, unfelt, by night, by day,
Abroad we take our airy way:
We foster love and kindle strife,
The bitter and the sweet of life:
Piercing and sharp, we wound like steel;
Now, smooth as oil, those wounds we heal:
Not strings of pearl are valued more,
Or gems enchased in golden ore;
Yet thousands of us every day,
Worthless and vile, are thrown away.
Ye wise, secure with bars of brass
The double doors through which we pass;
For, once escaped, back to our cell
No human art can us compel.