32 AGATE 



pyrophyllito (hydrous silicate of alumina), whilst others aro silicates of magnosia, J 

 either hydrous or anhydrous. 



AGAR-AGAR. A seaweed forming a large article of commerce in the East. 

 It is frequently called Bengal Isinglass, from the fact of its being found largely in 

 the Bengal market. It is used for making jellies and for stiffening purposes. See 



AGAZtXCTTS. A genus of the class Fungi, so numerous that 4,000 species 

 have been enumerated. The mushrooms are of this order. The Agaricus cam- 

 pestris is the one commonly used in this country as food, and from which the 

 sauce called ketchup is made. In Italy this species is considered poisonous, while 

 many species used there and in France are unused here. The truffle is a mushroom, 

 Tuber ciharium, and its commercial value is so great that in Home the yearly average 

 of taxed mushrooms from 1837 to 1847 was between 60,000 and 30,000 Ibs. weight. 

 The Agaricu* muscaria is a poisonous species, though used by the natives of 

 Kamtschatka and Korea to produce intoxication ; the Kussian name is monchomore, 

 and an infusion of this taken with some liquor produces raving delirium and not 

 unfrequently a desire to commit suicide or assassination. Another variety is 

 beautifully phosphorescent. The Agarici grow in decaying animal or vegetable 

 matter ; they are cellular plants, with a rounded tludlus on a stalk ; the spores or 

 seeds occur underneath the cup in the gills or hymenium. Their growth is remarkably 

 rapid, and there is often great difficulty in distinguishing between the edible and 

 poisonous varieties. 



All the fungi of this genus contain a larger amount of nitrogen than either peas or 

 beans. The following are a few of the analyses given in Watts's ' Chemical 

 Dictionary : ' 



Nitrogen. 



Agaricus deliciosus . . 4-68. 



arvensis . . 7'26. 



muscarius . . 6'34. 



Lycoperdon echinatum . . 6'16. 



Upon this depends their nutritive properties. 



AGATE. (Agate, Fr. ; Achat, Ger.) The term Agate is not employed to denote any 

 distinct mineral of uniform composition, but is applied rather to certain mixtures of 

 siliceous minerals, consisting of different varieties of chalcedony usually associated 

 with jasper, quartz, amethyst, and other natural forms of silica. The agate is the 

 axaTTjs of the Greeks, by whom it was so called after the river in Sicily of that 

 name (now the Drillo), whence, according to Theophrastus, agates were first procured. 

 Bochart, with much probability, deduces the name from the Punic and Hebrew, 

 nakad, spotted. 



In some agates, as in certain varieties from Saxony and Bohemia, the chalcedony 

 and other component minerals have been deposited in fissures, thus forming true 

 veins ; but in by far the greater number of cases the materials of the agate have been 

 formed, layer after layer, in the cavities of a vesicular rock. When the formation of 

 the agate has proceeded with regularity, a transverse section of the stone exhibits a 

 number of concentric lines representing the edges of the successive deposits these 

 deposits differing one from another in colour, density, and other physical characters, 

 and thus producing the variegated patterns exhibited by most agates. As the com- 

 ponent minerals are formed in regular sequence, the successive layers being deposited 

 from without inwards, it follows that the innermost portion of an agate must always 

 be the most recent. 



Agates are usually found either embedded in a rock called inelaphyre, or in the form 

 of free nodules, liberated by decomposition of the matrix. Although the term 

 melaphyre has been somewhat loosely applied, it is now generally used to designate a 

 fine-grained eruptive rock, composed mainly of a felspar either oligoclase or labrado- 

 rite and augite, with more or less magnetic iron-ore : it is chiefly associated with 

 stratified rocks of palaeozoic age. When fresh, the melaphyro, as its name implies, 

 (/utXas, black) is usually of a black or very dark colour, but on weathering it often 

 becomes green or brown ; it is the altered varieties of melaphyre that most commonly 

 contain agates. Some varieties are porphyritic, and were formerly called augitic 

 porphyry. 



Unwilling to admit the igneous origin of the agate-bearing melaphyres, some 

 authorities, as Bischof, have maintained that the cavities now occupied by 

 siliceous minerals have been formed by the removal of crystals in the porphyritic 

 rock these cavities having been enlarged and their angles rounded off by subsequent 

 solution. Although such an explanation may be admissible in certain cases, it seems 

 much more probable that in by far the larger number of rocks the cavities were 



