As we have observed before, the boiling point of a liquid depends upon the pressure to which it is subjected. Under the normal atmospheric pressure of 14.7 pounds per inch the boiling point of water is 212 degrees F., but if the pressure be increased the boiling point rises, and if it be reduced the boiling point is lowered. In a partial vacuum of ten pounds absolute pressure the boiling point is 193.2 degrees, at one pound it is 102.1 degrees, and if the pressure is reduced to .089 pound water will boil at 32 degrees, or its normal freezing point. Dr. Cullen, by exhausting the air from a vessel containing water, made the water boil or vaporize at a low temperature. In order to boil it had to absorb heat, and not being supplied with any external heat it had to draw upon itself, thus producing ice.
[Illustration: FIG. 80.--DIAGRAMMATIC VIEW OF A VACUUM REFRIGERATING MACHINE]
Dr. Cullen’s machine has been improved upon by using various chemical substances to absorb the water vapor. Such a machine is shown in Figure 80. The vacuum chamber _A_ is partly filled with brine, which may be cooled below the freezing point of pure water without congealing. A pump, _B_, maintains a vacuum in the chamber. In the upper part of the vacuum chamber there is a vessel, _C_, into which sulphuric acid is sprayed from a reservoir, _D_. This acid has a strong affinity for water vapor and hastens the evaporation by absorbing the vapor with which it comes in contact. The mixed sulphuric acid and water flows over into a receiver, _E_. The acid is reconcentrated by steam heat so that it can be used over again. However, this feature of the process is not shown in the diagram. Brine from the chamber _A_ passes through a coil of pipe _F_ in the tank _G_, where the ice is made, and it is returned to the vacuum chamber by an injector _H_, which at the same time introduces fresh water into the chamber to take the place of that absorbed by the acid. The fresh water and brine enter as a spray at _I_, so as to increase the rate of evaporation.
[Illustration: FIG. 81.--ORIGINAL ABSORPTION MACHINE]
Another form of refrigeration is known as the absorption system, and strangely enough direct heat is applied to the machine in one place in order to abstract heat from it in another. A diagrammatic representation of the first machine of this type (which was invented by Frederick Carré) is shown in Figure 81. Two vessels, _A_ and _B_, are employed, which are connected by a tube _C_. The vessel _A_ contains ammonia solution. A lamp, _D_, is placed under the vessel _A_. Sufficient heat is produced to vaporize the ammonia, whose boiling point is very low, and distill it out of the water in the solution. It passes through tube _C_ into vessel _B_. This vessel is surrounded by a tank, _E_, containing cold water, which condenses the ammonia vapor into liquid ammonia, then the process is reversed.