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 

 ma}^ 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. {Algm FalmellacecB). 

 — Probably a resting form of Euglena. See 



EUGLENA. 



ANiECTANGIUM, 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 phsenomena 

 scarcely need description, since analytic cry- 

 stals merely play the part of a thick plate of 

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

 ized light be transmitted through them (a 

 polarizer alone being used),, in one position 

 they suffer it to pass freely, while if they are 

 rotated 90° they arrest or absorb it entirely, 

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

 of selenite, or other depolarizer, be placed 

 beneath the slide upon which the crystals 

 are situated (without the analyser), the 

 lateral surfaces are seen to be coloured, the 

 complementary tints appearing at each quar- 

 ter rotation. 



Of com'se these crystals mil act equally as 

 polarizers and analysers. Mr. Talbot gives the 

 following explanationof the cause of the phse- 

 nomena in the crystals which he examined. 

 When a beam of polarized light is transmitted 

 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 of 

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

 edge will appear green. On reversing the 

 polarization of the light, these colours are ex- 

 changed. This observation shows why the phse- 

 nomenon only occiu"s in crystals possessing 

 strong double refraction, like nitre, in which 

 the refractive indices of the two rays are mate- 

 rially different. When a ray of common light 

 is incident upon such a crystal, and therefore 

 is divided into two rays oppositely polarized, 

 both rays are transmitted through the cen- 

 tral parts of the crystal, which are bounded 

 by parallel planes, or by planes approaching 

 to parallelism. But when the bounding 

 planes of the crystal are much inclined to 

 each other, and therefore refract the light in 

 the manner of a prism, the refractive indices 

 of the rays may differ so much, that while 

 one passes freely through such a ])rism, the 

 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 the 

 other. Therefore if two oppositely polarized 

 rays are presented to such a crystal as in our 

 experiment, one will be transmitted and the 

 other not. That this is the true explanation 

 appears from this, that when the oblique 

 planes are well-formed and clearly defined by 

 the microscope, the colour also is accurately 

 limited by the same boundary ; so that while 

 this part analyses the tints of a plate of sul- 

 phate of lime, the rest of the crystal is in- 

 active. 



