IXTEODUCTOBT. 15 



is nncombined. So, also, nitrogen and oxygen are com- 

 bined in nitric oxide, but uncombined in the air, as that is 

 a mere mixture of these gases. Some elements, as you will 

 see in a future chapter, are never found in an nucombined 

 state, but are obtained in this state only by processes in the 

 laboratory of the chemist. 



7. Variety in their Combinations. There is very great va- 

 riety in the combinations of many of the elements, in form, 

 in color, and in other qualities more essential than these. 

 You will hereafter learn, in Chapter VI, that nitrogen and 

 oxygen form five combinations very different from each 

 other. And then one of these compounds, nitric acid, forms 

 a vast variety of combinations with many of the metals. 

 Take the compounds of mercury as another example. Oxy- 

 gen forms with it two oxides a gray oxide and a red oxide. 

 Sulphur also forms with it two compounds one a black 

 powder, and the other black also till it is sublimed, and 

 then it ia red, and constitutes the pigment called vermil- 

 ion. Besides these, there are various compounds of mercury 

 with nitric acid, sulphuric acid, etc. As we proceed with 

 our investigations in future chapters this variety will be 

 developed to you, and the examples which I have given 

 will suffice for the present. By far the greatest variety, as 

 you will see, is shown in organic substances. Here, for the 

 most part, there are only four elements, sometimes but three, 

 as stated in 407. With these few elements, what an end- 

 less variety of forms, colors, odors, tastes, and other quali- 

 ties is. presented by vegetable and animal substances ! 



8. Difference in Form between Mineral and Organized Sub- 

 stances. The forms which the combinations of the elements 

 assume in organized or living substances are very different 

 from those which they have in substances which are not liv- 

 ing. In the former the tendency is to curved lines, but in 

 the latter, with few exceptions, to straight lines and angles. 



