I started reading it some years ago but not when it first started, so it's been less than 10 years. I've re-read bits of it from time to time; and there have been occasional references to Dyne water later than the section I linked to.
I had to hunt for that first reference for a while; but I knew it had to be somewhere in the sequence between when Agatha first entered the Castle and when it got frozen in time; and not really late in the sequence, because she discovered that power source before she discovered how to finish powering the Castle.
If you scroll down to near the bottom of the Girl Genius pages, there's a white box with the name of the page in it. If you click on that, you can scroll through all the pages by volume and name. (Well, almost all -- it seems to stop in early November 2021. Maybe it'll catch up eventually.) The names are fairly cryptic descriptions of what's in the page, but they do give some idea, and when you don't recognize the pages you're after by their names you can probably get some idea of where you are in the story if you read through a few of them.
It does look rather like Dyne water, doesn’t it? And being part of what makes the Heterodynes what they have become, it is hard to forget. I do wonder whether it is a component of Jagerdraught, which would explain much about them. Dunno if that was ever mentioned in the story?
On the page right after the one I linked, it says it is indeed one of the ingredients of Jagerdraught. I wonder if the rest of the ingredients aren't things intended to improve the chances of survival.
