The advent of higher pressures and temperatures in the modern steam generating plant has made necessary a better understanding of the chemical phenomena associated with the production of steam under these new conditions. This is true not only in the large electricity generating plants where the production of steam receives a degree of attention comparable with that given the accumulation and distribution of its one product electrical energy; but it is true, also, in steam plants of both large and small manufacturing organizations where plant design is showing a marked tendency to keep pace with the development of the art. Boiler scale, the steam producers' greatest enemy, has now to be fought in a new way. The weapons used are not new, but a new appreciation of the potentialities of those weapons exists, whereby, of necessity, the old rule-of-thumb methods of treating water to prevent scale in low pressure boilers has given way entirely to complete scientific treatment based upon the chemistry of the changes that the water and its soluble salts undergo under steaming conditions in the more modern higher pressure boilers. Associated with the problems of scale prevention are those of embrittlement and corrosion, both of which have new aspects in modern steam generation. In the lower pressure type of boiler where the water, either treated or untreated, caused a precipitation of scale on the inner surfaces of the drums and tubes, corrosion was usually not serious. If it developed, little damage was done, and often a corrosive condition was not discovered until the units were dismantled to make room for new equipment. In many of the modern high pressure boilers, (by high pressure we have reference to operation above 250 pounds per square inch gage) the feed water is scientifically treated to prevent scale, embrittlement,