426 FIRST CLASS OF MINERAL CONSTITUENTS. 



SILICA. 



As the skeleton of the vertebrate animals chiefly owes its 

 hardness to the phosphate of lime whicli it contains, and the shell 

 of the invertebrate animals to the carbonate of lime, so the shields 

 of the lowest classes of animals are rendered hard and firm by con- 

 taining a large quantity of silica. This substance is so thickly 

 deposited in these organs that neither decomposition nor incinera- 

 tion can destroy their form; hence it is that deposits of fossil infu- 

 soria are so often discovered. 



Silica for the most part occurs only as an incidental constituent 

 of the juices and tissues of the higher classes of animals; Gorup- 

 Besanez* has, however, shown by numerous experiments that this 

 body forms an integral constituent of feathers and of hair. 



Small quantities of silica have also been found in the blood, in 

 the white of egg, in the bile, in urine, and in the solid excrements, 

 and occasionally in certain morbid concretions. 



The Bacillarice are the most remarkable of all the infusoria in 

 relation to the quantity of silica which they contain ; their shields 

 equally resist the action of fire and of acids. We are indebted to 

 Ehrenbergf for our first accurate knowledge on this subject, and 

 for the discovery of fossil infusoria in flint, mountain meal, &c. 



Henneberg,J as well as Gorup-Besanez, has determined the 

 quantity of silica in feathers ; the latter, however, has fully investi- 

 gated the subject in all its bearings, and extends his enquiry to the 

 determination of the influence exercised by species, age, food, and 

 other circumstances on the deposition of silica in the feathers. 



Gorup generally found from O'l 1 to 2'4/ir of silica in the feathers 

 of different birds, and from 6*9 to 65'0 of silica in the ash. The 

 last-named quantity, which was the largest he ever found, occurred 

 in the feathers of Perdix cinerea,but the feathers of Striv flammea, 

 Gallus domesticus, and Corvus frugilegus, yielded ashes very rich in 

 silica. The feathers of granivorous birds contained from 1 '69 to 

 3'7l-- of silica (and their ash yielded from 25*5 to 50g-) ; the fea- 

 thers of birds living on fish and aquatic plants contained on an 

 average 0'23, and their ash 10'5 of silica; those of birds living on 

 flesh and insects yielded, as a mean, 0'64, and their ash 27 ; and 



* Ann. d. Ch. u. Pharm. Bd. 66, S. 321-342. 

 t Die Infusionsthierchen u. s. w. S. 143-169. 

 Ann. d. Ch. u. Pharm. Bd. 61, S. 255-61. 



