88 ANSWERS TO PRACTICAL QUESTIONS 



cles of silica will take its place atom by atom and thus pet- 

 rify the wood. The wood has not been changed to stone, but 

 has been replaced by stone. 



12. In what part of the body, and in ivhat forms, is 

 pJiosphorus found ? 



As a phosphate it is tho principal earthy constituent of the 

 bones. It is also a never-failing ingredient of the brain and 

 nervous system. The susceptibility of phosphorus to oxidation 

 especially adapts it to the rapid changes incident to the struct- 

 ure and offices of the brain.* 



* Phosphorus is an element which can imperceptibly and quickly pass 

 from a condition of great chemical activity to one of equal chemical inert- 

 ness. In virtue of this character, it " may follow the blood in its changes, 

 may oxidize in the one great set of capillaries, and be indifferent to oxygen 

 in the other; may occur in the brain, in the vitreous form, changing as 

 quickly as the intellect or imagination demands, and literally naming that 

 thoughts may breathe and words may burn ; and may be present in the 

 bones in its amorphous form, content like an impassive caryatid, to sustain 

 upon its unwearied shoulders the mere dead weight of stones of flesh. And 

 what is here said of the brain as contrasted with the bones, will apply with 

 equal or similar force to many other organs of the body. All throughout 

 the living system, we may believe that phosphorus is found at the centers 

 of vital action in the active condition, and at its outlying points in the 

 passive condition. In the one case it 13 like the soldier with his loaded 

 musket pressed to his shoulder and his fmger on the trigger, almost antici- 

 pating the command to fire ; in the other it is like the same soldier with 

 his unloaded weapon at his side standing at ease." 



" Further, phosphorus forms with oxygen a powerful acid, capable even 

 of abstracting water from sulphuric acid, and yet perfectly unirritating 

 to the organic textures. Taking up varying quantities of water, phosphoric 

 acid assumes no fewer than three distinct forms, which will unite with 

 one, two, or three atoms of alkali respectively, giving an acid, neutral, or 

 alkaline reaction. Thus it is available for the most varied uses in the 

 body. A child is beginning to walk, and tho bones of its limbs must be 

 strengthened and hardened ; phosphoric acid, accordingly, carries with it 

 three units of lime to them, and renders them solid and iirir. But the 

 bones of its skull must remain comparatively soft and yielding, for it has 

 many a fall, and the more elastic these bones are, the less will it suffer 

 when its head strikes a hard object ; so that in them we may suppose the 

 phosphoric acid to retain biit two units of lime, and to form a softer, less 

 consistent solid. And the cartilages of the ribs must be still more supple 

 and elastic, so that in them the phosphoric acid may be sxipposed to be 



