REFRIGERATION. 790 REFRIGERATION. before it is mixed with the salt. The Siemens apparatus has been used in making artificial ice ■with much success, but, being less economical than more modern ice-machines, has never come into general use. When these devices or others of the same type are used for cooling purposes brine is cooled in them and then circulated in the usual manner through a system of circulating pipes. The general law governing the production of cold by frigerific mixtures is that during the liquefaction of a solid a certain amount of heat not indicated by or sensible to the thermometer is absorbed, which heat is abstracted from any surrounding bodies. The absorption of heat from the surrounding bodies is the greater the more rapidly the solid is liquefied. Vacitum Process. The cooling of liquids on this principle depends upon the conversion of the sensible heat into latent heat (see Heat) during evaporation, and lias been practiced in all ages. A primitive example of this process in its crudest form is the practice in India and other warm coimtries of placing earthen vessels of water in a natural or artificial draught so that the liquid may be cooled by surface evaporation. The first machine for tlie production of ice by the vacuum process appears to have been invented in 1755 by Dr. Cullen, who in that year made the dis- covery that the evaporation of water could be facilitated by the removal of the atmospheric pressure by means of an air pump, to such a degree as to enable him to freeze water even in summer. This apparatus was the parent of all those subsequently designed, but seems not to have been a commercial success. In 1777 Naime found that by the introduction of sulphuric acid into the receiver for the exhaust the aqueous vapor could be absorbed from the rarefied air and th^ latter dried, thvis preventing the forma- tion of a permanent atmosphere over the water and hindering the continuity of the evaporation. Nairne was followed by other inventors, but it was not until the second quarter of the nine- teenth century that Edouard Carre invented a commercially successful machine adapted to pro- duce the carafes frappes commonly used in Paris- ian cafes and restaurants. This machine con- sisted of a cylindrical vessel intended to contain the charge of concentrated sulphuric acid, of an air pvmip so arranged that it could be connected to the mouth of the carafe, and of an agitator cdupled to the air-pump lever for the purpose of keeping the sulphuric acid in motion. The Carre machine proved most successful for its purpose, and improved forms of the device are still manufactured, the largest of which are capable of producing 80 pounds of ice per day. In 1878 Franz Windhausen patented a vacuum machine, an improved form of which was in- stalled in 1881 at the Aylesbury Dairy, London, England. This machine was nominally capable of producing from 12 to 15 tons of ice per 24 hours. The ice-forming vessels in this machine were six in number, circular in transverse sec- tion and slightly tapered. The mouths of these vessels were connected with the sulphuric-acid chamber and the vacuum pump, and water was admitted to them in fine streams, which offered extended surfaces for evaporation and almost instantly congealed into ice globules which fell to the bottoms of the molds and there froze together. To facilitate the release of the ice from the molds, they were surrounded with hollow jackets into which steam could be forced until the ice was melted loose. Various other forms of vacuum machines have been devised, but the two described are typical examples and explain the process sufliciently. Like the liquefaction process, the vacuum process cannot compete in economy with the more strictly mechanical processes, and vacuum machines are now used onty for domestic ice-making and similar small installations. Compression Process. The system of absorb- ing heat and thus producing cold partly by vap- orization and subsequent liquefaction, and partly by compression and cooling, is in accordance with the well-known law of physics that all substances during the process of passing from a liquid to a gaseous state are bound to absorb a certain amount of heat, and while returning from a gase- ous to a liquid state to give up or throw off the same amount of heat. Whatever the refrigerat- ing or heat-absorbing agent that may be used, the following cycle of operations is obligatory in all machines woi'king upon this principle : ( 1 ) Compression, that is the refrigerating agent in gaseous form is subjected to a pressure sufficient to reduce it to a liquid form, this pressure vary- ing with the nature of the agent and the tempera- ture of the condensing water. During this com- pression a degree of heat is developed in accord- ance with the amount of pressure to which the gas is subjected or to the volume to which it has to be reduced relatively to that of the gas in order to produce liquefaction. (2) Condensa- tion, during which process the heat developed during the compression of the gas is carried away by forcing the latter through water-cooled pipes, the heat being transferred to the cooling water. At this point the gas is ready to assume the liquid form, in doing which an additional amount of heat is given off to the water. (3) Expansion, during which the liquefied gas is admitted to series of coils of pipe, and being suddenly relieved of pressure, instantly flushes or expands into gaseous form, in doing which, according to the above mentioned law of physics, it is forced to take a quantity of heat which it FlO, 1. DIAGRAM ILLrSTRATING THE OPEHATION OP A RE- f RIOEBATINO MACHISE OH THE COMPBE88ION PBINCIPl.E. draws from the surrounding objects, first, of course, the pipes wherein it is confined, and second, such substances as may be in contact with the pipes and which it is desired to cool, as air, water, and brine. The amount of heat thus abstracted or absorbed is equal to that previously given up to the cooling water in the condenser. This cycle of compression, condensation, and e.x- pansion having been completed, the refrigerating agent is in its first state again and is ready for another cycle. The tliree operations described being essential, all machines operating according
