The ancients made a kind of mortar so very hard and binding that it is now found to be almost impossible to separate the parts of some of their buildings. The lime used in these harder mortars is said to have been prepared from the very hardest stones, sometimes from marble. Fine sand makes weak, and coarse sand strong mortars, and the sand should be washed before mixing, to obtain the large grains. The lime should be thoroughly burned, and perfectly white. The principle on which it hardens is, that the lime absorbs carbonic acid from the air, and hardens, forming a concrete round the grains of sand. It was customary to mix with the lime and sand chopped straw, but cow's hair has been substituted ; this is only introduced to cause it to bind together, and prevent cracking in the drying. It is only used for the prime coat.