Formalin has two purposes in these films.
1. Cross link the gelatin and harden it as Jerry says.
2. Stabilize the dye images by reacting with the excess coupler to form a "neutral" environment for keeping.
Pre hardener formula:
Sodium Sulfate 100 g/l
Formalin 3 - 10 ml /l
Sodium Carbonate 10 - 50 g
pH 9.0 - 9.5
Treat for 1 - 5 mins at the working temp then wash 3 - 5 mins.
Make up Photo Flo 200 at the working strength as on the bottle.
Add 3 - 10 ml of Formalin to 1 L of this.
Use for 1 - 2 mins at the working temp.
There is NO substitute for Formalin (37% Formaldehyde gas in water) for these purposes).