This is the second of a series of three lectures on stone in architecture. It describes the characteristics of various kinds of building stone and the agencies that contribute to disfiguration and decay. The most important of these are: 1. Atmospheric pollution (smoke and sulphur fumes); 2. Contamination with soluble salts; 3. Frost. Stone preservatives are discussed and details are given of experiments and trials conducted to determine the best means of preventing deterioration of stonework. Treatments with paint, wax, limewash, magnesium silico-fluoride solution,silicone ester and silicone resins are included. Methods of cleaning by washing, or with steam, and the technical aspects of the restoration of old stonework are dealt with. Data on the testing of stone covers freezing (exposure) tests and crystalization tests. Information is given regarding the physical and chemical composition of certain classes and varieties of stone in relation to their weathering qualities and laboratory methods for investigating them.