This lead to the development of the solid block vial (as fitted to the longer C.K and Avit levels). As it's name suggests, it is made from a solid block of virtually unbreakable Plexiglass.
How they are made only affects how strong they are, not the principle of how they work. In fact they both work in exactly the same way. Quite simply when the bubble settles exactly between the two marker lines (called graticule rings), the level is 'true'.
The bubble floats in a coloured spirit derived from oil. Spirit is used because of its low freezing point, −114°C, which prevents it from freezing in cold weather.
So, the vial is the most important part of a spirit level. But as with all things, technology marches on.
As previously mentioned, the original vials consist of a simple curved tube, however, whilst they are fine for use on pocket levels, they are a little too vulnerable to damage, particulary where they are liable to get a lot of knocks; for instance in bricklaying.
Most spirit levels have at least two vials - one for indicating how horizontal (level) or how vertical (plumb) a surface is.