The Glass Phial is a magical version of the Glass Bottle. While it is used at first to craft Scribing Tools, its main function is to hold raw Essentia.
"Glass phials are capable of holding all manner of liquids and substances, but the most important to the thamaturge is their ability to hold Essentia. Essentia is not easy to gather however and it indelibly stains the phial, making it only useable once."
Each glass phial can hold 8 Essentia of a single aspect of magic , meaning you cannot mix different aspects in the same phial.
In order to fill a glass phial with Essentia, simply right click on an Arcane Alembic that has 10 Essentia in it.
Glass phials that have been filled with Essentia have many practical uses. They can be used for research, or thrown into crucibles for crafting purposes.
If you right click on a Warded Jar with a filled phial, it will fill the jar with it. Jars hold up to 64 Essentia of a single aspect. It takes 8 phials to fill a Warded Jar. This is useful for storage purposes, or to fuel the Essentia cost