I remember Glass Trial Jars at school in the Chemistry lab. I had no idea they were really used for measuring specific gravity and alcohol content with a hydrometer in wine and beer homebrew! If I’d known then what I know now I would definitely have paid more attention in class! and maybe have achieved a top grade G.C.S.E. or “A” Level in Chemistry for budding home-brewers! Tip: Glass trial Jars are "smashing" wheras Plastic Trial Jars are equally effective for this purpose, cheaper and less likely to be smashing!