Evaporating Dishes or Watch Glasses are glass, concave, circular lab dishes that are used for a variety of purposes in the laboratory. These 150mm diameter dishes are designed to survive being heated, so they can be used as a small evaporation surface. They are often used as lids for beakers and larger flasks, to keep dust and other contaminants out during experiments. They are also used to hold samples for low power microscope observation. The name, Watch Glass, comes from them originally being used as covers for pocket watches, before finding other uses in laboratories.