76 ELEMENTARY PHYSIOLOGY 



Nitrogen, forming about 2 '5 per cent, of the weight. 

 Calcium 1-3 

 Phosphorus 1-2 ,, 



Sulphur 15 



Sodium, chlorine, fluorine, potassium, iron, magnesium, and 

 silicon are also present in small quantities. 



Each of these is present in combination with others, thus giving 

 rise to a variety of chemical compounds ; but oxygen, nitrogen, 

 and hydrogen also exist in small proportions in the elementary form 

 as gases. 



Thus we see that the body is made up of a number of chemical 

 compounds, each compound being formed by the union of two or 

 more elements. These compounds are the proximate principles 

 which enter into the formation of the various tissues of the body. 



Before proceeding to describe the chief compounds of the 

 body, we will study the general characters of its four chief elements, 

 viz. oxygen, carbon, hydrogen, and nitrogen. 



The simple experiment described above is sufficient to illustrate 

 the chief characteristics of oxygen. It is an invisible gas, which 

 powerfully supports combustion or burning, but is not itself com- 

 bustible. Oxygen forms about one-fifth of the volume of the atmo- 

 sphere, and is that constituent of the air which supports combustion 

 and animal life. It also forms eight-ninths of the weight of water, 

 and is so abundant in the earth's crust that it is calculated to 

 form more than one-third the weight of the whole globe. 



Carbon is a solid element which is capable of existing in a 

 variety of forms. As a natural mineral substance we are familiar 

 with it in two forms black lead (plumbago or graphite) and the 

 diamond; the latter being its purest and crystalline form. Charcoal 

 and lamp-black are artificial varieties of carbon. When carbon 

 burns, it combines with the oxygen of the air, forming an invisible 

 compound gas, called carbonic acid gas. This gas is also produced 

 by the combustion of any compound substance which contains the 

 element carbon, such as coal, coal gas, wood, oils, spirits, &c. 

 (See page 78.) 



In order to ascertain the chief properties of hydrogen gas, it 

 will be advisable to resort to a few simple experiments : 



(a). Fit any kind of bottle with a good cork, through which passes a 

 straight glass tube ; and see that the apparatus is air-tight with the exception 

 of the tube through which the gas has to pass. Put into the bottle a few 

 scraps of zinc, and cover these with diluted sulphuric acid (about eight parts 

 of water to one of the strong acid). Hydrogen gas immediately begins to 

 come off, and, being lighter than air, may be collected in an inverted bottle. 



(b). Collect some of the gas in a dry bottle as above described, and then 

 hold the mouth of the bottle near a flame, taking great care that no flame is 

 brought near the apparatus in which tke gas is being generated. The hydrogen 



