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] THE ABSORPTION PROCESS 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. The circulation of water through tank _E_ is checked and water from pipe _F_ is sprayed on vessel _A_. This cools the contents of vessel _A_, producing a partial vacuum. The ammonia in chamber _B_ boils and its vapors pass back into chamber _A_, where they are reabsorbed by the water in that chamber. The rapid vaporization in chamber _B_ absorbs heat from the immediate surroundings and will freeze water placed in the tank _E_.