What is the function of boiling stones in a fractional distillation?

Answer

Bye Milo

You use cooking stones to let the liquid boil regularly. The use of cooking stones is neatly explained on Wikipedia, take a look there.

Then your question about the factions:
In a distillation you bring a mixture of liquids to a boil. The most volatile substance will boil first. This is the substance with the lowest boiling point. Often this is also the fabric with the lowest density.
The light substances distil first, followed by the heavier substances. In the course of the distillation, for example, the composition of the distillate will change and the mass and volume fractions are no longer the same.
The separation of petroleum into the different fractions, for example, is based on this principle: petrol, diesel, motor oil, … (see the link).

Kind regards