The capacity of the material to hold heat is know as its Thermal Mass – in this case materials used for solar heating. Commonly cement, water, stone and oil are used.
In temperate and desert regions of the planet, stone has been used since time immemorial for Solar Heating and solar lighting. During the day the stone absorbs the heat into itself, keeping the interiors cool and during the night they radiate this heat back into the atmosphere, the modern Solar Heating uses this principle for modern homes too.
What is exciting is that this same Solar Heating principle can be used to keep the homes warm in the colder countries. The placement, size and material used along with other factors like daylight, shade, etc; need to be applied properly for the Solar Heating to work to the optimum. When properly designed, the thermal mass maintains the ambient temperatures in a comfortable range..