AIR VESSELS ALABASTER. 



Some of the orehideoui tribe are also entitled to 

 the appellation of air plants. They constitute the 

 greatest vegetable ornament of the tropics. 



Air plants, though frequently brought to this 

 country, have only lately been cultivated with any 

 degree of success. 



Aericles arachnoicles. 



AIR VESSELS. A name given to the spiral 

 vessels of plants, or vessels with a spiral coat which 

 run longitudinally round the centre or pith of the 

 young branches of trees or shrubs. They are well 

 seen in the alder and lilac. These vessels have been 

 proved to convey the sap which serves for the nou- 

 rishment of the plant, but they would appear occa- 

 sionally to contain air, sometimes with a greater 

 proportion of oxygen than exists in the atmosphere. 

 This led some physiologists in former times to regard 

 them as true air vessels furnishing the air contained 

 in the leaves. The leaves, however, have been 

 shown not to depend on these vessels for their supply 

 of air. 



AITONIA( Linnaeus). A small green-house under- 

 shrub, native of the Cape of Good Hope. This solitary 

 plant was named in honour of the late W. Aiton, Esq., 

 of the royal gardens, Kew. Linnaean class and order, 

 Monadelphia Decandria. Natural order, Meliacece. 

 Generic character : calyx, monosepalous, four cleft ; 

 corolla, of four petals ; stamina, eight, protruding ; 

 the iilaments united in one brotherhood, inserted un- 

 der the ovarium ; style, filiform, tipped with an obtuse 

 stigma. Fruit, a berry, containing several seeds. 



AIZOON (Linnaeus). A small genus of five spe- 

 cies, under-shrubs and herbs kept in green-houses as 

 ornamental plants. Linnaean class and order, Icosan- 

 dria Pentagynia. Natural order, Ficoidece. Ge- 

 neric character : calyx, monosepalou?, five-parted, 

 and persistent ; corolla, none ; stamina, arranged in 

 groups of threes in the angles of the calyx ; styles, 

 five, on a pentangular capsule of five places, and 

 opening by as many valves. 



ALABASTER. There are two kinds of stone to 

 which this name is commonly applied. The first of 

 these, the carbonate of lime, was much used by the 



ancients. It is nearly as hard as calcareous spar, 

 and its specific gravity about 2,700. 



This species is found encrusting the roofs, walls, and 

 floors of caves, particularly those situated in limestone 

 rocks. It is formed from water holding carbonate of 

 lime in solution ; and when the water first escapes 

 into the air, it is capable of holding in solution a large 

 quantity of that earthy body ; but when the solution 

 comes to be agitated, or exposed to the atmosphere, 

 or to a change of temperature, the carbonic acid 

 makes its escape, and thus deprives the water of its 

 solvent power. Water thus impregnated with car- 

 bonate of lime oozes slowly through the fissures of 

 the rocks : there some time elapses before a drop of 

 sufficient size to fall by its own weight is formed, and 

 in this interval some of the solid particles are separated 

 from the water, owing to the escape of the carbonic 

 acid,' and adhere to the roof. In this manner, suc- 

 cessive particles are separated, and attached to each 

 other, until a stalactite is formed. If the percolation 

 of the water containing calcareous particles be too 

 rapid to allow time for the formation of a stalactite ; 

 the earthy matter is deposited from it after it has 

 fallen from the roof upon the floor of the cave, and 

 in this case the deposition is called a stalagmite. In 

 some cases, the separation of the calcareous matter 

 takes place both at the roof and on the floor of the 

 cave ; and in the course of time, the substance of 

 each deposition increasing, they both meet, and form 

 pillars, often of great magnitude, and which appear 

 to have been originally destined to support the roof 

 of the cave. 



Caves of this kind occur in almost every country. 

 Macalister's Cave, in the island of Skye, and those 

 in the limestone hills of Derbyshire, are the most 

 striking appearances of this kind hitherto observed in 

 our island. But the most celebrated stalactitic cave 

 is that of Antiparos, in the Archipelago, which has 

 been particularly described by Tournefort. Similar 

 caves occur in Germany, France, Switzerland, Spain, 

 in the United States of America, and other countries. 



In many places springs at a high temperature rise 

 out of the ground, of a greyish white colour, occa- 

 sioned by a quantity of gypsum, or chalk, which they 

 hold suspended in a state of half-solution. In pro- 

 portion as these grow cool, and loose their carbonic 

 acid, the earthy particles are deposited, leaving the 

 channels through which they flow covered with a 

 compact alabaster, of a dazzling white colour. 



The most remarkable spring of this description in 

 Europe is situated on a mountain, near Radicofani. 

 It supplies the baths of St. Philip in Tuscany, and 

 forms the source of the little river Paglia. Advan- 

 tage has been occasionally taken of this circumstance 

 to obtain very beautiful impressions of bas-reliefs, by 

 exposing the moulds to a current of such water, till 

 they have become filled with the earthy deposit. 

 The hardness of the alabaster depends upon the 

 degree of obliquity at which the mould is placed to 

 receive the dashing of the water. The more vertical 

 the position, the harder the alabaster. The hardest 

 models not being so white as the softer, the water is 

 frequently caused to make a circuitous course before 

 it reaches the mould, that all the grosser particles 

 may be previously deposited. Even the softer ones, 

 however, are as hard as Carrara marble, and surpass 

 it in whiteness. The time required for these produc- 

 tions varies, according to the thickness, from one 

 month to four. When the mould is sufficiently filled 



