The importance of safety in any scientific endeavor is never in question. However, when cryogenic temperatures are involved, safety is especially important. In addition to observing the normal precautions, one must also take into account the variations of physical properties that occur at low temperatures. At these tempera- tures, some properties not only exhibit large differences from their normal values but also can vary widely over a small temperature range. Before any cryogenic project is started, a thorough knowledge of the possible hazards is necessary. Only in this way can the safest...