This clue isn't actually an ingredient. It is a formula on how to put the other 38 ingredients together so that they can be assembled properly.
The Serum FormulaEdit
For Gideon Cahill's serum:
One portion = one ounce
Start with one portion of water.
Add 1/8 portion of each of the ingredients on the list given to Luke Cahill.
Add 1/16 portion of each of the ingredients originally told to Jane Cahill.
The rest of the formula is unknown.