148 NON-METALS AND THEIR COMBINATIONS. 



We thus see that, while in physical properties O and H resemble 

 one another closely, their chemical properties are practically the re- 

 verse of each other. Elements which, like the metals, combine readily 

 with oxygen, do not combine with hydrogen ; and, vice versa, ele- 

 ments which, like chlorine, combine most readily with hydrogen, will 

 scarcely combine with oxygen. It will be shown later that, as a 

 general rule, elements which resemble one another in chemical prop- 

 erties are not apt to combine with one another, while those differing 

 widely have great affinity for one another. 



Nascent hydrogen. It was stated above that hydrogen is a good reduc- 

 ing agent, but as far as hydrogen in the free state is concerned reduction takes 

 place, as a rule, only when heat is employed. There is a condition of hydrogen, 

 however, in which it is able to reduce many compounds at ordinary tempera- 

 ture, while free hydrogen has no measurable action on the same. For exam- 

 ple, the hydrogen liberated during electrolysis of a dilute acid is able to reduce 

 many compounds present in solution immediately around the pole (cathode) at 

 which the hydrogen is produced. It also shows different degrees of activity 

 according to the material of which the pole is made. Similarly, hydrogen 

 generated by the action of dilute acids on metals has reducing power on sub- 

 stances immediately surrounding the metals during action, whereas free 

 hydrogen gas passed through a solution of the same substances or in contact 

 with them in the dry state has no action. To illustrate : hydrogen gas passed 

 through a solution of arsenous oxide, As 2 3 , has no effect, but if the oxide is 

 present in a mixture of dilute hydrochloric acid and zinc the hydrogen formed 

 quickly reduces it to arsine gas, AsH 3 . This is one of the most delicate tests 

 for arsenic. The more active condition of hydrogen at the time of its libera- 

 tion is spoken of as the nascent state. It seems that this increased activity of 

 hydrogen in contact with the substances that liberate it is an example of con- 

 tact or catalytic action (see page 154). Good support to this view is the fact 

 that the efficiency of the nascent hydrogen varies according to the nature of 

 the material in association with which the hydrogen is produced. 



Water, H 2 O = 17.88. Hydrogen monoxide. Water exists on 

 our globe in the three states of aggregation. Air at all temperatures 

 contains water in the gaseous form. Liquid water occurs plentifully 

 in the oceans, rivers, etc., and also in plants and animals. Seven- 

 tenths of the human body is water; potatoes contain of it 75 per 

 cent, and watermelons as much as 94 per cent. Solid water exists 

 not only as ice and snow, but it also enters into the composition of 

 many rocks, and is a constituent of many crystals containing water 

 of crystallization. 



Absolutely pure water is not found in nature. The purest natural 

 water is rain-water collected after the air has been purified from 

 dust, etc., by previous rain. Comparatively pure water may be 



