990°, or, to use round numbers (for minute accuracy is not here our object), 1000°. It follows, therefore, that an ounce of water, in passing from the liquid state at 212° to the state of steam at 212°, receives as much heat as would be sufficient to raise it through 1000 thermometric degrees, if that heat, instead of becoming latent, had been sensible. (57.) In order to derive all the knowledge from these experiments which they are capable of imparting, it will be necessary to examine very carefully how water comports itself under a variety of different circumstances. If water be boiled in an open vessel, with a thermometer immersed, on different days, it will be observed that the fixed temperature which it assumes in boiling will be subject to a variation within certain small limits. Thus, at one time, it will be found to boil at the temperature of 210°; while, at others, the thermometer immersed in it will rise to 213°; and, on different occasions, it will fix itself at different points within these limits. It will also be found, if the same experiment be performed at the same time in distant places, that the boiling points will be subject to a like variation. Now, it is natural to inquire what cause produces this variation; and we shall be led to the discovery of the cause, by examining what other physical effects undergo a simultaneous change. [Pg109] If we observe the height of the barometer at the time of making each experiment, we shall find a very remarkable correspondence between it and the boiling temperature. Invariably, whenever the barometer stands at the same height, the boiling temperature will be the same. Thus, if the barometer stands at 30 inches, the boiling temperature will be 212°. If the barometer fall to 29-1/2 inches, the thermometer stands at a small fraction above 211°. If the barometer rise to 30-1/2 inches, the boiling temperature rises to nearly 213°. The variation in the boiling temperature is, then, accompanied by a variation in the pressure of the atmosphere indicated by the barometer; and it is constantly found that the boiling point will remain unchanged, so long as the atmospheric pressure remains unchanged, and that every increase in the one causes a corresponding increase in the other. (58.) From these facts it must be inferred, that the pressure excited on the surface of the water has a tendency to resist its ebullition, and to make it necessary, before it can boil, that it should receive a higher temperature; and, on the contrary, that every diminution of pressure on the surface of the water will give an increased facility to the process of ebullition, or will cause that process to take place at a lower temperature. As these facts are of the utmost importance in the theory of heat, it may be useful to verify them by direct experiment.