IJCHII) Cin'STALS. 



15y I.. J()i;i.lN(".. i;.Sr.. A.K.C.SC. 

 With llliistrat'uins fntni f^holoi^ni/ylis kiiiilly lahcii hy Professor l.chnuiiiii. 



\\'iii:n it wiis announced nioro tlian a coiipk' ol 

 licradcs af^o tliat snUstances had Ix'cn discovori-d, 

 which, though li(Hiid, cxliihitcd thu properties usually 

 ass<iciated with crystals, an outliurst of adverse 

 criticism was the immediate and not surprising 

 result. After the first stir had subsided, the attitude 

 became one of unreasoning sce|)ticism : the observa- 

 tions themselves were discredited and the disturbing 

 conclusions calmly ignored. Time, however, has 

 favourably modified the trend of scientific opinion, 

 though even to-day, particularly in England, the 

 importance of the subject is not fully realised, and, 

 in consequence, but slight attention is devoted to it. 

 Yet the phenomenon is such a curious and surpris- 

 ing one that whatever interpretation be put upon it, 

 a brief account of its jirincipal aspects cannot fail to 

 bo of interest. 



After all, only a little consideration is sufficient to 

 demonstrate the reasonableness of the idea of 

 crvstallinitv in the liquid state. On tlie orthodox 

 view, a substance which is on the point of 

 undergoing crystallisation has its molecules moving 

 about in the haphazard way which is characteristic of 

 an ordinary Huid, but immediately the temperature 

 at which crvstallisation occurs is reached, these are 

 supposed to arrange themselves in a definite order, 

 according to the symmetry of the crystal to be 

 formed. Such a conception, involving as it does 

 the sudden formation of cosmos out of chaos, is 

 difficult of comprehension. It must be admitted 

 that it would be far more rational to imagine that, 

 in a liquid as it nears the crystallisation temperature, 

 a inar^halling of the molecules is taking place w hich 

 reaches its culmination at the moment of separation 

 of the solid crystalline form. In other words, the 

 possibility of crystallinity in the liquid state must be 

 conceded. Let us see how far experimental cnidence 

 bears out this deduction. 



EXPERIMKNTAI. EVIDENCE. 



The discovery of liquid crystals will always be 

 associated with the name of Lehmann, for it was he 

 who stumbled upon the first example, and it is 

 largeh' due to his persistent activity that we owe the 

 rapid development of the subject. 



In 1.S76, in a series of experiments with his 

 " cr\stallisation-niicroscope," to which attention w ill 

 soon be turned, Lehmann observed that silver iodide, 

 though exhibiting a hexagonal form at the ordinary 

 temperature, changed at 146" into a cubic modifica- 

 tion, which was not only plastic but actually liquid. 

 While still dubious as to the exact significance of 

 this isolated instance, Reinitzer in 18.S8 drew 

 Lehmann's attention to a substance, cholestervl 



i)en/()ate, which exhibited a double melting-point : 

 that is to say, on heating, the solid melted at a 

 defitiiti: temperature, and this, on further heating, 

 suddenly clarified. The intermediate turbid phase 

 Lehmann found to be at once mobile and doubly 

 refractive ; so that taking this in conjunction with his 

 own example, he at once declared for the possibility 

 of the existence of what he has called " liquid- 

 crystals." 



Since then.exampleshaveturnedupmore frequently 

 tliaii might have been anticipated, so that there are 

 now nearl\- three hundred compounds which can be 

 brought into the same categorvascholesteryl benzoate. 

 These compounds are all organic and of very diverse 

 structure: but instincts of compassion, as well as 

 considerations of space, spare the infliction upon the 

 reader of any but the simplest of the weird and 

 wonderful names with w hich thev have been labelled. 



("KVSTALLiSAT II )\ -Microscope. 



IScforc jiroceeding further, some notice must be 

 taken of the instrument to which we owe the 

 discovery of and subsequent investigation upon 

 " liquid-crystals." 



A " cr\stallisation - microscope" is shown in 

 Figure 44.5. The instrument consists of an ordinary 

 microscope, which is provided with means for raising 

 a substance to any desired temperature, for main- 

 taining it there and for cooling it rapidly to another 

 temperature. The heating is effected by means of a 

 miniature Bunsen burner, A, capable of being swung 

 into position below the centre of the stage. A 

 delicate adjustment, B, comprising a lever moving 

 over a graduated arc, is provided for the regulation 

 of the height of the flame, and by the use of an air- 

 blast, not shown in the figure, the bunsen is con- 

 vertible into a blow-pipe. Also fitted to the 

 instrument are one or more cooling-blasts, C, 

 mounted usually upon universal joints and fitted 

 with an arc adjustment, D, by means of which the 

 liquid upon the slide can be lowered in temperature 

 at almost any desired rate. It is thus possible, bv a 

 suitable combination of both heating and cooling, to 

 conduct a microscopic examination of a substance 

 for quite a long time at a constant temperature. In 

 the modern complicated instruments, the arrange- 

 ment of the parts is slightly different from that 

 shown in the figure, whilst such additions as water- 

 jackets for the lenses and electric connections on the 

 stage are provided. 



Let us now consider any one of the manv well- 

 known substances which yield " liquid-crystal " 

 droplets and follow its behaviour under the micro- 

 scope. .\ little of the substance — usually in some 



