AMEIRA. [ c 



AMEI'R A, Boeck. A genus of Copepoda 

 (Entoinostraca). 



A. longipes. Marine. North Britain. 



BIBL.' Boeck, Overs. Norges Copepoder , 

 1864, 49 j Brady, Copepoda (Hay Soc.), ii. 

 36. 



AMI'BA, Duj. See AMCEBA. 



AMMONIA, HYDROCHLORATE OR MU- 

 RIATE OF. This salt crystallizes in cubes, 

 octahedra, and trapezohedra. When crys- 

 tallized rapidly it forms curious feathery 

 aggregations (PI. 11. fig. 9). The crystals 

 do not polarize light. 



AMMONIA, OXALATE OF. This salt is 

 readily prepared by neutralizing a solution 

 of oxalic acid with ammonia or its carbo- 

 nate, and evaporating. 



It crystallizes in long slender needles, 

 belonging to the right rhombic prismatic 

 system. When mounted in Canada balsam, 

 these form a very beautiful object for the 

 polariscoipe (PI. 39. fig. 7). 



AMMONIA, OXALURATE OF; formerly 

 known as the lithoxanthate of ammonia. 



This salt may be prepared by mixing 



1 part of uric acid with 32 parts of water, 

 and heating the mixture in a porcelain cap- 

 sule until it acquires a boiling temperature. 

 Strong nitric acid, previously diluted with 



2 parts of water, is next added in small 

 quantities at a time, until nearly the whole 

 of the uric acid is dissolved. The liquid is 

 then boiled, filtered, mixed with excess of 

 solution of ammonia, and concentrated by 

 evaporation. As it cools, the salt is depo- 

 sited in needles or warty groups of crystals. 

 These are freed from the mother-liquor by 

 pressure between blotting-paper, dissolved 

 in warm water, and a little solution of am- 

 monia added. On evaporation the pure salt 

 separates. 



The oxalurate of ammonia forms one of 

 the most beautiful and interesting sub- 

 stances that can be examined by the polar- 

 izing microscope. When a small quantity 

 of its aqueous solution is slowly evaporated 

 on a slide, some of it usually crystallizes in 

 circular crystalloid disks or very flat spheres, 

 consisting of minute needles radiating from 

 a centre and in an intimate state of mecha- 

 nical adhesion ; sometimes the extremities 

 of the needles are seen projecting beyond 

 the circumference of the disks. The latter 

 appear colourless or yellowish by reflected 

 light ; pale or dark brown or even black by 

 transmitted light, according to their size and 

 thickness. When immersed in Canada bal- 

 sam, they become transparent, often nothing 



| AMMONIA. 



more being distinguishable than radiating 

 lines, indicating the needles of which they 

 are composed. But if examined by polar- 

 ized lignt and with the analyzer, when 

 these are so arranged that the plane of po- 

 larization of the analyzer is at right angles 

 to that of the polarizer (the field being 

 black), the disks present the appearance of 

 beautiful little stars, sometimes almost 

 white, at others splendidly coloured, each 

 being also traversed by a black rectangular 

 cross (PI. 39. fig. 11). 



On rotating the slide, no change is pro- 

 duced. But on rotating the analyzer or 

 polarizer 90, the arms of the cross appear 

 to rotate, which, as there are no fixed points 

 visible in the disks, gives rise to the ap- 

 pearance of the disks themselves rotating. 

 When the analyzer has Been rotated a quar- 

 ter of a revolution, the former position of the 

 black cross is occupied by a white one, and 

 the colours of the intermediate parts become 

 complementary to (forming white light 

 with) those which they at first possessed, 

 these appearances being alternately repro- 

 duced at each quarter revolution. 



If a plate of selenite is placed beneath 

 the slide, the beauty of the objects is much 

 augmented (PI. 39. fig. 12). On some parts 

 of the slide, dendritic aggregations of the 

 needles are seen (PI. 39. iig. 11 a). 



Sometimes the colours are disposed in 

 concentric rings ; when these are well de- 

 fined, a concentric arrangement of the 

 groups of needles is distinguishable on 

 examining the disks by common light. 



A simple experiment will show the origin 

 of the cross and the colours. If eight crys- 

 tals of any doubly refracting salt be ar- 

 ranged upon a slide in the directions of 

 equidistant radii of a circle, they may be 

 regarded as forming two crosses, alternating 

 in position. If the slide be placed under 

 the microscope, with the plane of polariza- 

 tion of the polarizer and analyzer at right 

 angles, and the crystals be simultaneously 

 rotated and kept in the same relative posi- 

 tion, a point will be reached at which each 

 alternate crystal will become black, the in- 

 termediate ones appearing coloured ; and on 

 continuing the rotation, the crystals which 

 were at first black will appear coloured, 

 those which were coloured appearing 

 black. 



The blackness of the crystals arises from 

 the plane of primitive polarization of the 

 light transmitted by the polarizer being 

 parallel with the optic or neutral axis of the 



