This was definitely the most trouble-prone and vexing part of the air-cooled aircraft engine. All engine designers made serious efforts to improve exhaust valve reliability and durability. Experiments with internally cooled valves were started as early as 1913. Initially, water and mercury, as previously stated, were tried as the heat transfer agents in the sealed, hollow valve stems. While the material in the stems was almost always called a coolant, in actuality it was a heat transfer agent used to carry heat from the head of the valve to the stem where it could be passed through the VALVES guide and into the fins on the cylinder head. As mentioned earlier, water proved to be impractical and mercury did not work well because it did not wet the interior of the valve stem and therefore did not transmit heat well. During his early years at McCook Field the ever-ingenious Sam Heron had observed the characteristics of various sodium compounds which are normally used in heat-treating operations. These materials are solid at room temperature and become liquid at engine operating temperatures. He observed that since these compounds wet the surface of steel alloys readily and transfer heat very well, their use should be effective in extending the life of exhaust VALVES. The ancestor of our present-day sodium-cooled valves had arrived, thanks to Mr. Heron, and almost ninety years later we are still enjoying the benefits of his ingenuity though even today such valves are not completely fault free.