Silver has been known as a bactericide for over a thousand years. A colloid is quite simply a sub microscopic particle of one substance suspended in a different substance (the medium). Even in ancient times, it was known to prevent disease and was commonly thought that disease could not be transferred from one person to another if by drinking from a silver cup. Silver coins were dropped into milk to prevent spoilage and silver containers were used to store foods in general. Until almost 1970, it was common for scientists to sterilize petrii dishes by placing a silver coin in them and silver was long used for plates for the surgical repair of bones. The most common usage of silver was by members of royal families, who used silver utensils almost exclusively. The result was that over a number of generations, royalty enjoyed freedom from infectious diseases, unlike commoners, who ate from earthen ware jugs and bowls and who were frequently ill.