REFRACTION. 789 REFRIGERATION. mately as the tangent of the zenith distance down to a zenith distance of about 70°, and is entirely independent of the distance. The effect of re- fraction upon tlie ris- ing and setting of the heavenly bodies is to make them appear to rise earlier and set later than they would if there were no at- mosphere. Tile hori- zontal refraction var- ies from 35' to 39'. Therefore when we limb rising the whole the horizon. In this lower below way the refraction accelerates the sunrise in our latitudes from 2 to 4 minutes. At sunset the sun is delayed an equal amount. Thus the total effect of refraction is to increase the length of the day at the expense of the night from 4 to 8 minutes, according to the inclination of the sun's diurnal circle to the horizon. Refraction also explains the elliptic appear- ance of the sun's and moon's disks when near the horizon. Owing to the rapid increase in the amount of refraction near the horizon, the lower limb appears more elevated than the upper, thus shortening the vertical diameter. Another effect of refraction when the air is much disturbed is to make the stars 'dance,' especially when seen through a large telescope with high power. This is due to the constant displacement of the image by the varying re- fraction. REFRIGERANTS (from Lat. refrigeram, pres. part, of refrigerare, to cool again, from re-, back again, anew -f- frigerare, to cool, from frigus, Gk. pfyos, rliigos. cold). A name given to a class of medicines which reduce the tempera- ture of fevers, allay thirst, and cause a feeling of coolness and refreshment throughout the system. The}' lessen the force of the circulation by qiiiet- ing the ever-excited heart, and excite perspiration and an increased urinary flow. Most of these remedies belong to the. class of the vegetable acids and their salts, but aconite and similar drugs, which act primarily on the circulation, may also be considered refrigerants. The vege- table acids used are tartaric, citric, acetic, and oxalic. Citric acid is most useful in the form of lemonade or orange juice; acetic acid in the form of vinegar. Oxalic acid is a dangerous drug and is only given in very dilute solutions and in vei-y sniall doses. The diaphoretic re- frigerants ("those which excite perspiration) are aconite, veratrum viride, and potassium citrate. The latter is a favorite ingredient of fever mix- tures. The diurctie refrigerants (those which stimulate the urinary flow) are the bicarbonate, citrate, acetate, bitartrate (cream of tartar), sulphate, chlorate, and nitrate of potassium. The latter is active particularly in the form of the ethereal spirits popularly known as 'sweet spirits of nitre.' REFRIGERATING MACHINES. See Re- frigeration. REFRIGERATION (Lat. refrigeratio, from refriqcrare, to cool again) . The art of producing cold "by artificial means. It has been practiced since very ancient times, but it is only in com- paratively recent times that improved systems and apparatus have enabled such operations to be conducted profitably and on a commercial scale. Jleclianical refrigeration is now employed in the manufac^ture of artificial ice; for the freez- ing and chilling of freslily killed meat in slaugh- ter houses; for the cooling of stores for meat, fish, fowl, fruits, vegetables, and other perish- able provisions; for cooling the atmosphere of dwellings and hospitals; for certain engineering operations; and for a variety of manufacturing processes. The number and variety of refrig- erating devices available for these purposes are very great, but they all belong to one or the other of the following five classes : ( 1 ) Devices in Avhieh the more or less rapid liquefaction of a solid is utilized to abstract heat; (2) devices by which the abstraction of heat is effected by tii"e evaporation of a portion of the liquid to be cooled; (3) devices in which the abstraction of heat is effected by the evaporation of a sejiarale refrigerating agent of a more or less volatile nature, which agent is subsetpiently returned to its original condition by mechanical compression and cooling; (4) devices by which the abstrac- tion of heat is effected by the CT'aporation of a separate refrigerating agent of more or less vola- tile nature under the direct action of heat, which agent again enters into solution with a liquid; (.5) devices in which air or other gas is first compressed, then cooled, and afterwards i)er- mitted to expand w-hile doing work. These five processes of refrigeration are termed, respective- ly, the liquefaction process, the vacuum process, the compression process, the absorption process, and the cold-air process. LiQUEFACTio. Pkocess. Liqucfaction is one of the most ancient methods employed for arti- ficial cooling. The reduction of the temij^raturo of water by the melting of saltpetre is said to have been known in India at a very leniote period. The Romans are said to have cooled wine by immersing the bottle containing it in a second vessel filled with cold water, into which salt- petre was gradually thrown, while at the same time the bottle was rapidly rotated. Freezing water by the use of a mixture of snow or pow- dered ice and saltpetre was mentioned by I.atinus Tancredus in 1G07, and wine by means of snow and common salt by Santorin in lG2(i. The best among the many forms of apparatus for making ice on this principle are probably those of Toselli and Siemens. In Toselli's machine the frigerific agent employed is a mixture of ammonium ni- trate and water, which produces a reduction of temperature of about 40° F. The apparatus con- sists of a vessel in which the solution of the ammonium nitrate is effected, and of a can wherein are placed a number of circular molds of dill'ercnt sizes. These molds previously filled with water arc inserted in the freezing mixture and a thin film of ice is formed round their edges; these tapered tubes of ice are then with- drawn from the molds and placed one inside the other, thus forming a small stick of ice. In Siemens's apparatus the frigerific agent is c.nl- cium chloride, whose dissolution in water pro- duces a reduction of temperature of only 30° P.. and to admit of this reduction being sufficient to produce ice with water at an initial temperature of 0.5° F., a heat interchanger is provided in which the spent liquor, which is at a temperature of about 30° F., is employed to cool the water
