302 PHYSIOLOGY AT THE FAKM. 



The average proportion of bone-earth in this substance is 

 from 52 to 61 per cent. Maracaibo, or Monk's Island guano, 

 is American phosphate. The phosphoric acid, amounting to 

 41.34 per cent in this substance, appears to form a mixture of 

 pyro-phosphate and of tribasic-phosphate of lime, or bone- 

 earth ; whence it is rich above the average of such minerals 

 in phosphoric acid. Sombrero rock, or crust-guano, has been 

 brought of late years in quantity to Britain from the West 

 Indies. It is not a guano, but the rock itself, of which the 

 islet Sombrero is almost entirely composed. It appears to be 

 a rock breccia. It averages as much as 75 per cent of bone- 

 earth. Kooria Mooria (Curia Muria) guano is imported into 

 Britain in considerable quantities from the group of islands of 

 that name on the south-east of Arabia. It contains organic 

 matter and from 45 to 60 per cent of phosphates of lime 

 and magnesia. It occurs in a fine powder, and is accounted a 

 valuable manure. It thus appears that commercial specula- 

 tion has been by no means idle in ransacking the earth to 

 obtain a supply of phosphates for the uses of agriculture.* 



Silicium or Silicon. Silicon, one of the most abundant 

 substances in nature, is found in small proportion throughout 

 the organised kingdoms. In the animal kingdom a minute 

 proportion can commonly be detected in the bones and in 

 the urine. As respects its presence in the urine, probably 

 much depends on the kind of food employed. In the vege- 

 table kingdom it gives strength to the stem, as in grasses, so 

 as to enable it to support the weight of the grain. In the 

 bamboos of the East Indies a deposit of pure silica occurs in 

 considerable masses, which has been named " tabasheer." In 

 the stem of the equisetacese or horse-tails, which some animals 

 will eat, the silica is seen deposited in a crystalline arrangement. 



* On this subject see Voelcker, ' Agricultural Journal of England, ' vol. 

 xxi. p. 350. 



