AMYLUM. 



[ 32 ] 



ANALYTIC CRYSTALS. 



it swells into a clear jelly, which is at first 

 stiff, but gradually acquires liquidity ; alco- 

 hol or water throws down from it white flakes 

 of amyloid, which are coloured blue like 

 starch by iodine. It differs however from 

 starch, in the circumstance that the iodine 

 can be washed out of it, and the blue colour 

 made to disappear by the action of water, 

 which is not the case with starch. 



The production of this substance forms a 

 valuable test of the presence of cellulose 

 in the higher plants. In applying the test, 

 the tissue to be examined is first dyed with 

 aqueous solution of iodine, the excess is then 

 removed by a piece of sponge, and a drop of 

 strong sulphuric acid added, and the mixture 

 set aside. The time required for the pro- 

 duction of the colour will vary ; sometimes 

 it appears immediately, at others some hours 

 may elapse. In some cases it can only be 

 produced when the substance has been pre- 

 viously treated with solution of potash or 

 strong nitric acid. Care must be taken, in 

 making this experiment, that the slide be 

 perfectly clean; it should be wiped with a 

 silk handkerchief or piece of wash-leather, 

 and not a linen or cotton cloth, because these 

 consist of cellulose ; also, blotting-paper 

 should not be used to absorb the excess of 

 the solution of iodine, for the same reason. 

 It frequently happens that the sulphuric acid 

 precipitates the iodine from the solution in 

 the form of exceedingly minute crystals, 

 which in mass have a purplish-blue tint. 

 This should not be forgotten as a source of 

 fallacy. See CELLULOSE. 



AMYLUM. See STARCH. 



ANABAINA, Bory. See TRICHORMUS. 



ANACALYPTA, Rohl. (Musci). See 



POTTIA. 



ANACYSTIS, Kiitz. (Alga Palmellacea). 

 Probably a resting form of Euglena. See 

 EUGLENA. 



AN^CTANGIUM, Hedw. A genus of 

 Mosses. See PILOTRICHUM. 



ANALYTIC CRYSTALS. This term 

 was proposed by Mr. Fox Talbot, in 1837, 

 to designate those crystals which possess the 

 power of analysing polarized light, like the 

 tourmaline. The substances in which this 

 property is best exhibited are the nitrate of 

 potash, the sulphate of chrome and potash 

 dissolved in tartaric acid by heat, boracic 

 acid, the oxalate of chromium and potash, 

 allantoin, hippuric acid, urea, oxalate of urea, 

 uric acid, &c. They must be immersed in 

 Canada balsam. The crystalline compound 

 of disulphate of quinine with iodine is infe- 



rior to none in this power. The phaenomen 

 scarcely need description, since analytic cry 

 stals merely play the part of a thick plate c 

 tourmaline, or a Nicol's prism, i. e. if polar 

 ized light be transmitted through them ( 

 polarizer alone being used)., in one positioi 

 they suffer it to pass freely, while if they ar 

 rotated 90 they arrest or absorb it entirety 

 or to a greater or less extent ; and if a plat 

 of selenite, or other depolarizer, be place* 

 beneath the slide upon which the crystal 

 are situated (without the analyser), th 

 lateral surfaces are seen to be coloured, th 

 complementary tints appearing at each quar 

 ter rotation. 



Of course these crystals will act equally a 

 polarizers and analysers. Mr. Talbot gives th 

 following explanation of the cause of the phse 

 nomena in the crystals which he examined 

 When a beam of polarized light is transmitte< 

 very obliquely through a small prism of nitre 

 its outline generally exhibits two colours in 

 stead of one ; for while the edge of the prism 

 which is on that side from whence the ray o 

 light comes, is, for instance, red, the opposit 

 edge will appear green. On reversing thi 

 polarization of the light, these colours are ex 

 changed. This observationshows why the phse 

 nomenon only occurs in crystals possessinj 

 strong double refraction, like nitre, in whicl 

 the refractive indices of the two rays are mate 

 rially different. When a ray of common ligh 

 is incident upon such a crystal, and therefon 

 is divided into two rays oppositely polarized 

 both rays are transmitted through the cen 

 tral parts of the crystal, which are bounde< 

 by parallel planes, or by planes approachin{ 

 to parallelism. But when the boundinj 

 planes of the crystal are much inclined t< 

 each other, and therefore refract the light ii 

 the manner of a prism, the refractive indice 

 of the rays may differ so much, that whiL 

 one passes freely through such a prism, thi 

 other cannot pass at all, but suffers total in 

 ternal reflexion, and is thereby dispersed 

 just as if the prism had a larger refracting 

 angle with respect to that ray than to thi 

 other. Therefore if two oppositely polarize* 

 rays are presented to such a crystal as in ou 

 experiment, one will be transmitted and thi 

 other not. That this is the true explanatioi 

 appears from this, that when the obliqui 

 planes are well-formed and clearly defined b; 

 the microscope, the colour also is accuratel 1 

 limited by the same boundary ; so that whiL 

 this part analyses the tints of a plate of sul 

 phate of lime, the rest of the crystal is in 

 active. 



