One of the essences in Roper Klacks' laboratory that, when mixed, create his potions. Each one can be smelt, handled or listened to, in order to match it to the pictures in Klacks' spell book.
This is the essence of cobwebs. No one knows exactly why it is green. It smells of morning dew, feels like thin strands of hair, and 'sounds' like many tiny spider legs.
Obtained on the floor behind the curtains in the laboratory.