Apart from the bone corpuscles and the soft contents of the Haversian canals and of the spaces of the cancellated bone, the hard bony substance proper is composed of animal and mineral matters so intimately combined that the smallest distinguishable bit of bone contains both. The mineral matters give the bone its hardness and stiffness, and form about two-thirds of its weight when dried. They may be removed by soaking the bone in diluted muriatic acid,* and the animal or organic part of the bone is then left as a tough, flexible mass, retaining perfectly the shape of the original bone.

What radiate from the lacunú? Into what do the innermost canaliculi of an Haversian system open ? How is nourishing liquid from the blood carried throughout a bone ?

What lies in each lacuna of a living bone? What are the bone corpuscles ? What is the hard part of bone ? How is a whole bone made up ?

Of what primary constituents is bone composed ? Is there any fragment of bone that does not contain both ? What qualities do its mineral parts give to a bone ? How much of a dry bone consists of mineral matter ? How may the mineral parts be removed ?

* Add a couple of ounces of muriatic acid to a pint of water and place a sheep's rib in the mixture for four or five days, having previously scraped the bone quite clean. It will be found so flexible that a knot may be tied on it; the specimen may be preserved in strong brine or dilute alcohol from year to year for exhibition to a class.

When long boiled in water the greater part of the animal portion of bone is turned into gelatine and dissolved in the water; most of the gelatine which we buy in the shops is obtained by boiling fresh bones in a closed vessel under a high pressure; the water then gets much hotter than when boiled in the air, and dissolves out the gelatine more quickly; when a shin of beef is used to make soup the bones are put in as well as the softer parts, and the whole is kept boiling for hours so as to get some of the gelatine out of the bones. The animal matter of bone gives it its toughness and flexibility.

The earthy portion may be obtained free from the animal by calcining a bone in a bright fire. The residue is a white and very brittle mass, which retains perfectly the shape of the original bone. It is readily powdered and then forms bone ash, which consists chiefly of phosphate and carbonate of calcium; most of the phosphorus of commerce is obtained from it. If the burning be imperfect the animal matter is charred but not altogether burnt away, and a black mass, known as animal charcoal or "bone black," is left.

What then remains behind? What are its properties? Has it still the shape of the bone? What happens when a bone is boiled for hours? How is the gelatine of commerce obtained? Why do we use bones in making soup ? What properties does its animal matter confer on bones ?

How may we get the mineral part of bone free from the animal ? What are its properties when isolated ? What is bone ash ? From what is phosphorus prepared? What is animal charcoal?