NOVEMBKR. 1912. 



KN(nVLKDGE. 



417 



suitable solvent in order to get isolated crvstals — is 

 placed between two cover-glasses on the microscope 

 stage and a very gentle heat is applied b\- carefully 

 regulating the height of the small bunsen llame 

 placed in position beneath the object-glasses. When 

 a clear melt or solution has been obtained, the bunsen 

 is swung out of action and the air-jet then brought 

 to bear uixin the slide. In order to observe the 

 formation and development of " li(juid-cr\stals,"' it 

 is best to rely upon their optical properties. To this 

 end, the nicols are crossed and the field of \iew 

 carefully observed. When a temperature sufficienth' 

 near the temperature of 

 crystal formation is 

 reached, pomts of light 

 appear in different parts 

 of the field and these 

 gradually increase in size 

 until discs of light are 

 attained, in each of which 

 black crosses can be ob- 

 served. If the plate be 

 now touched with a needle, 

 the slight pressure is 

 sufficient to cause dis- 

 tortion and as they regain 

 their original shape when 

 the pressure is removed, 

 there remains little doubt 

 as: to their liquidity. The 

 appearance of the field at 

 this stage is that of a 

 number of cross-imprinted 

 discs of light resembling 

 wheels, standing out upon 

 a dark background. This 

 continues until the tem- 

 perature has fallen suffici- 

 ently low to have reached 

 thetemperature of transition 

 of the liepiid into the solid 

 state, when the beautiful 

 prfsmatic colours, indica- 

 tive of the attainment of tlu' hitter condition, 

 quickh' make their appearance and re()iace the above 

 phenomena. 



Figure 444 shows the appearance of the field when 

 crystal drops of para-azoxyphenetol in olive-oil are 

 viewed in natural light. A similar Held when 

 observed in polarised light is shown in Figure 445. 

 where the dichroism' is clearh- shown, the two 

 colours being yellow and white. The next photo- 

 graph, Figure 446. illustrates fairly well the aspect 

 of the field of the same substance as seen between 

 crossed nicols. 



A remarkable instance, worthy of special mention, 

 is to be found in ammonium oleate, which Lehmann 

 investigated in 1894. By crystallisation of this 

 substance from alcohol, crystals separate which, 

 notwithstanding their fluidity, form well-defined 

 bi-pvramids, with edges more or less rounded. 

 Figure 447 gives some idea of these regular shapes. 



That they are really liquid can be tested in the usual 

 w ay by gently pressing the cover-glass. When two 

 of the bi-pyramids approach one another, they 

 arrange themselves at a certain angle and then slowly 

 coalesce to form a larger single crystal. Yet another 

 noteworthy property lies in their power of growth, 

 since if a crystal be broken in two, each part grow s 

 again at the expense of the substance still in solution, 

 and becomes a perfect crystal. 



Figure 448 shows another instance, in this case 

 para-azoxy benzoic acid ethyl ester, in which the 

 crystals, though ii(iui(l. reveal a definite geometric 

 form. The field is viewed 

 in ordinary light and the 

 crystals are shown in the 

 act of flowing together. 



There are even one or two 

 exceptional instances where 

 a substance has been dis- 

 covered which exhibits a 

 perfectly definite structure 

 bounded by plane faces and 

 sharp angles. In other 

 cases, and these are now- 

 becoming quite common, 

 dimorphism makes its ap- 

 pearance ; that is to say, 

 the substance exhibits two 

 liquid - crystalline phases 

 and therefore three definite 

 melting-points or, more ac- 

 curately, transition -points. 

 These dimorphous phases 

 are occasionally rendered 

 evident by their differing 

 degree of turbidity, though 

 usually they are different- 

 iated b}- their viscosities 

 or other physical properties. 

 Remarkable instances of tri- 

 and even tetra-morphism 

 have recentlv come to light ; 

 but naturalK' such cases 



Figure 443. 

 iiiiple form of Crystallisation Microscope. 



are very rare. 



An interesting phenomenon is to be observed when 

 " liquid-crvstals" are subjected to the influence of a 

 magnetic field : for under these circumstances the 

 drops rearrange themselves with their principal axes 

 in the direction of the lines of force. This is well 

 shown in Figures 449 and 450, where the former is 

 a reproduction of a photograph of crystal drops of 

 para-azoxv anisol in piperine in natural light, whilst 

 F"igure 450 represents the same drops after the 

 magnetic field has been set up. The lines of force 

 are in the direction of sight, and judging from the 

 dark points which have now appeared at the centres, 

 the drops have arranged themselves with their 

 principal axes perpendicular to the paper. 



Influence of the Chemic.\e Coxfigur.\tiox. 



A verv noteworthy feature of the liquid-cn,-stalline 

 condition is that it seems to be associated almost 



