AGAVE. 



[ ^8 



I 



J 



AIR. 



milky-white, in others the whole of the fibres 

 are impregnated with it. PI. 19, fig. 14 re- 

 presents sections of a piece of agate, show- 

 ing the silicified fibres of sponge, a; the 

 gemmules are seen at 6 ; a separate fibre at 

 c, and spicula at d. 



There are two distinct points connected 

 with the presence of these supposed organic 

 remains in agate; one is, whether they really 

 are organic remains, and the other is whether 

 they are related to the formation of the 

 agate, or merely accidentally present. The 

 first point is a very difficult one ; we have 

 only the microscopic appearance of the 

 bodies under one set of conditions to judge 

 from : this is always very unsatisfactory ; 

 many of the appearances most peculiar to 

 organic bodies, especially when the latter are 

 not connected so as to form a tissue, can be 

 closely imitated by crystallization. Still the 

 mass of evidence is decidedly in favour of 

 the appearances really representing portions 

 of sponges. 



In regard to the second point, we believe 

 that the necessary connexion of the presence 

 of the sponges with the formation of agate 

 cannot be maintained; but with this ques- 

 tion we have nothing to do. 



The supposed vegetable structures of 

 agates, described by Tm-pin, Miiller and 

 many others, have been clearly shown by 

 Prof. Goppert to be entirely inorganic pro- 

 ducts, chiefly dendi-itic deposits of oxide of 

 iron. His essay contains an elaborate history 

 of the strange notions which have at various 

 times been propounded concerning these 

 objects. Sometimes agate contains crystals 

 of quartz, carbonate of lime, or other mineral 

 matters imbedded in its substance. Those 

 paler varieties of quartz, which consist of 

 concentric layers of radiately grouped cry- 

 stalline needles, frequently polarize light very 

 beautifully. 



BiBL. Bowerbank, Trans. Geol. Soc. 1840, 

 {A7m. Nat. Hist. vol. vii. 1841 ; vol. x. p. 9 

 and 84) ; Toulmin Smith's objections (but 

 they refer rather to flint), Ann. Nat. Hist. 

 vol. xix. p. 1 and 89 ; Goppert, On the 

 plant-like bodies enclosed in Chalcedony, 

 Ratishon Flora, 1848, p. 57. See Flint. 



AGAVE.— See Fibres, Vegetable. 



AGRION. — A genus of Neuropterous 

 Insects. See Libellulid^. 



AINACTIS, Kiitzing.— A genus of Oscil- 

 latorieous plants growing on stones in water. 

 The two known species have been found in 

 Britain. 



1. A, granuUfera. Fronds from 1-12 to 



\-2" in diameter, often confluent, formed of 

 repeatedly dichotomous filaments, dark olive 

 green, containing separate particles of car- 

 bonate of lime. Rivularia granuUfera, Carm. 

 Hassall, Brit. Fr. Algce, Ixv. 1.4; Ainactis 

 alpina, Kiitz. Tab. Phyc. vol. ii. pi. 63. 1. 



2. A. calcarea, Kiitz. Fronds 1-4 to 1-2" 

 in diameter, orbicular, convex, ultimately 

 confluent, sometimes greenish, often dark 

 chestnut, composed of dichotomous filaments, 

 at length incrusted continuously with car- 

 bonate of lime. Kiitzing, I. c. pi. 63. ii. ; 

 Rivularia calcarea, Carmichael ; Lithonema 

 calcarea, Hassall, I. c. tab. Ixv. fig. 2. 



Kiitzing states that the gelatinous sheath of 

 the filaments of ^. alpina have a spiral fibrous 

 structure. See Spiral Structures. 



AIR. It need scarcely be remarked that 

 the air consists essentially of a mixture of 

 two gases, oxygen and nitrogen, in the pro- 

 portion by volume of about 21 parts of the 

 former to 79 of the latter, with variable 

 quantities of gaseous carbonic acid (about 

 1 -2000th) and aqueous vapour. Now as the 

 component molecules of gases are invisible 

 with any powers of the microscope, the air 

 possesses no microscopic characters. In two 

 respects, however, the study of the ah* in its 

 relations to the microscope is of great im- 

 portance : — 1st, in regard to the optical 

 appearances produced by the passage of 

 light through it when contained in bodies 

 submitted to microscopic examination ; and 

 2ndly, in regard to the particles which are 

 always, in greater or less numbers, suspended 

 in it. 



In microscopic investigations we meet with 

 air either existing in cells or cavities in various 

 tissues, or in the form of bubbles, confined 

 by the liquid in which the objects are usually 

 immersed. When surrounded and confined 

 by liquid, it mostly assumes a spherical 

 form, in accordance \^'ith the law of hydro- 

 statics, that the pressure of fluids is equal in 

 all directions ; sometimes the spherical form 

 is exchanged for that of a compressed or 

 oblong spheroid, the result of the pressure 

 of the glass slip covering the object. When 

 confined in cells or cavities, it assumes the 

 form of these. It is in general easily recog- 

 nized by transmitted light, from the smooth 

 and even darkness or shading given to its 

 margins, whilst in the centre it appears 

 luminous and clear. Sometimes the dark 

 margins of air-bubbles have a pale purplish- 

 yellow, blue or greenish tinge. By reflected 

 light, of course no darkness is produced, but 

 it then appears vitreous and shining, in con- 



