620 



INFRACTION 



REFRIGERATION 



yellow, while it absorbs the rest. Further, it is 

 'found that in these cases of anomalous disper-ioii 

 tlit* substance geiiei.illy has in the solid form a 

 surface-colour diHeri'iit from that seen through ita 

 solution ; and there are always absorption-Minds, 

 on the red aide of which the nmngibility is 

 increased, while on the other side it is diminished, 

 as if the molecules themselves took up oscillations 

 of particular periods and hurrie<l on the propaga- 

 tion of slightly slower or retarded that of slightly 

 more iapi.1 oscillations of the ether. It appears as 

 if this kind of art ion were never wholly absent ; the 

 spectrum produced by a prism never wholly coin- 

 cides with the diffraction spectrum in which the 

 deviation for each wave-length depends directly 

 upon the wave-length itself; and the spectrum 

 produced by a prism say of crown-glass does not 

 exactly coincide in its visihle distribution of colours 

 with a spectrum of equal length mode by a tlint- 



6 'ass prism. This is called the Irrationality of 

 ispersion. If now we take two prisms, such as C 

 (crown-glass) and F (flint-glass) in fig. 5, and pass 

 a beam of light through ; then, if the 

 angles of these prisms be suitable, the 

 rays dispersed by the one will be col- 

 lected by the otner, and there will on 

 the whole be deviation without disj>er- 

 sion ; but not absolutely so, on account 

 of the irrationality of dispersion of both 

 pi i-nis, the effect of which is that a cal- 

 culated ratio of angles and refractive 

 indices which will cause deviation with- 

 out dispersion for any given pair of wave-lengths 

 will, to a very slight extent in most cases, fail to 

 do so for the other wave-lengths present in the 

 mixed light transmitted through the system. By 

 the use of three prisms three wave-lengths may 

 similarly be achromatised. 



DOUBLE REFRACTION. The wave-surface de- 

 veloped when a disturbance originates at a point 

 in a homogeneous medium, like glass, is spherical in 

 form. In uniaxial crystals (see CRYSTALLOGRAPHY) 

 the di-t urbanee travels with two wave-fronta, one 

 spherical, the other ellipsoidal ; and the two wave- 

 fronts are coincident along the direction of the 

 optic axis, of such crystals some are positive, 

 such as quartz and ice, and in these the sphere 

 encloses the ellipsoid : in negative crystals, such as 

 Iceland -par ami tourmaline, the ellipsoid encloses 

 the sphere. If then a beam of light, plane-fronted, 



FiK.fi. 



Fig. 6. 



fall upon a slice of Iceland spur, the disturbance 

 at any point such as A (fig. 6) is transmitted from 

 that |M)irit in two |M>rtions ; one ixtrtion is refracted, 

 according to the principles of fig. 2 in article 

 REFRACTION, an an ordinary refracted ray. O : the 

 other u refracted in a way deteiiniriahle by using 

 In the coiutrnction, instead of 'the spheroid or arcs 

 of a circle, the corresponding ellipsoid, or arcs of 

 the appropriate ellipse, and it gives rise to the 



extraordinary refracted ray, E. The radius of the 

 smaller circle is to that of the greater as 1 -.ft ; the 

 tangent to the greater circle, at right angles to 

 XA, .111- SS in T; tangents TO' and TE' to the 

 smaller circle and the ellipse are also drawn so as 

 to pa through T ; the ray XA is deflected so as 

 to pass through the points at which these tangents 

 touch these curves ; and thus there are two re- 

 fracted rays, and an eye towards OE will see two 

 images of X. The light in the ordinary ray O is 

 found to lie polarised (see POLARISATION) in a 

 plane containing both the incident ray and the 

 crystalline axis : the extraordinary ray E is polar- 

 ised in a plane at right angles to this. In binaxial 

 ery-tals the three optical axes are dissimilar, ami 

 the wave-surfaces become complex : there are two 

 refracted rays. If a doubly refracting substance lie 

 put between two crossed Isicol's prisms (see POLAR 

 ISATION), light passes; and bv this means it is 

 found that many substances ordinarily not double 

 refracting become so when exposed to unequal 

 stress, as by pressure, heat, or rapid cooling. 



CONICAL REFRACTION. In certain cases light, 

 passing as a single ray through a plate of a biaxial 

 crystallised body, emerges as a hollow cone of rays ; 

 and in others a single ray, falling on the plate, 

 1 < -i ime- a cone inside the crystal, and emerges as a 

 hollow cylinder. These extraordinary appearances 

 were predicted from the wave theory of light by Sir 

 W. R. Hamilton (q.v.), and experimentally realised 

 by Lloyd. See Preston's Theory of Light (1890). 



Refrigerants ' are remedies which allay thirst 

 and give a feeling of coolness,' although they do 

 not in reality diminish the temperature of the 

 body. The following are the refrigerants in most 

 common use for internal administration : water, 

 barley-water, dilute phosphoric or acetic acid, 

 citric and tartaric acids taken in combination with 

 bicarbonate of potash as effervescing draughts, ripe 

 grapes, oranges, lemons ( in the form of Lemonade, 

 q.v.), tamarinds, chlorate of potash (ten grains 

 dissolved in water, and sweetened with svrup, to 

 be taken every third or fourth hour), ami nitrate of 

 poia-h. which may be taken in the same manner as 

 the chlorate, or as nitre-whey, which i- prepared by 

 boiling two drachms of nitre in a pint of new milk ; 

 the strained milk may be given in frequent doses 

 of two or three ounces. 



Refrigeration. In refrigerating machines 

 there is a transference of heat from the sub- 

 stance which is to be refrigerated to the cooling 

 agent, which is evaporating fluid, expanding gas, or 

 a material which promotes evaivoration of the liquid 

 to be cooled. If 80-025 pound-Centigrade unit- of 

 heat be withdrawn from a ponml of water at C. 

 it will become a pound of ice at the same tempera- 

 ture. If this heat 1- withdrawn from the water 

 by an evaporating liquid there are two conditions 

 which must be fulfilled ; the evaporating liquid 

 must evaporate very rapidly, and the latent heat 

 of evaporation (i.e. "the heat absorlied from outside 

 during evaporation) must be as great as possible. 

 Kther boils at 35 '5 C. (95'9 F.), and has at C. 

 (32 F.) a vapour-pi r-siire of 18'4 cm. (7 '36 inches) 

 of mercury; at Or C. it requires 04 Ib. -Centigrade 

 units of heat to evaporate a pound of it; and at 

 that temperature its evaporation ought accordingly 

 to lie able, if the whole of the heat required for 

 evaporation were withdrawn from water, to freeze 

 94 -f 80-025 times its weight of water at C., so 

 that a ton of ice (2240 Ib.) would lie produced by 

 the evaporation at C. of a minimum of 1907 Ib. 

 of ether. Alcohol is more advantageous than ether 

 in respect of its higher specific heat, but is pre- 

 ponderatingly leas so in respect of its lesser vola- 

 tility. Liquid ammonia boils at -35 C. ( -31* 

 F.), and has at 0" C. a vapour-pressure of 318 cm. 



