124 CHEMISTEY. 



been made, which acts much like a true alloy, and confirms 

 the view that hydrogen is a metal. 



157. How Compounds and Mixtures Differ. We have al- 

 ready stated the proofs that air is a mixture. Having now 

 become acquainted with the composition of water, you readi- 

 ly see, in regard to air and water, the two grand distinctions 

 between compounds and mixtures. 1. A chemical compound 

 differs wholly in its character from either of its constituents, 

 while a mixture does not. Water is entirely unlike either 

 the oxygen or the hydrogen that compose it ; but air is in 

 many respects like the oxygen and nitrogen, which are its 

 chief ingredients, having a mixture, as we may say, of the 

 qualities of the two gases. So, also, is the difference strik- 

 ingly illustrated by contrasting air with nitric oxide, as has 

 been shown in 85. 2. A compound always contains pre- 

 cisely the same proportions of its elements ; but in a mixt- 

 ure the proportions may be made to vary more or less. 

 Thus water always contains the same proportions of oxy- 

 gen and hydrogen ; but you can take away a part of the 

 oxygen of the air and increase its carbonic anhydride 

 ( 126), and it will be air still, though not good and health- 

 ful. 



158. Water as a Chemical Agent. Though water is a 

 very mild substance, and not powerful like the acids, it has 

 a great deal to do with the chemical operations every where 

 going on, as you will see as we proceed with the investiga- 

 tion of other subjects. It is the common solvent of the 

 world, dissolving, as you have already seen, gases, as well as 

 liquids and solids. It unites, as you will learn in 159, chem- 

 ically with many substances, being incorporated intimately 

 with them as water. Some solids can not exist in a crystal- 

 line form without having a certain amount of water in them, 

 and this is said to be their water of crystallization. Then 

 in the vegetable kingdom water is .decomposed to a consid- 



