506 REPORT OF NATIONAL MUSEUM, 1890. 



of the earth's crust. These sixteen, arranged according to their chem- 

 ical properties and the order of their abundance, are as follows: 



Metalloids Metals. 



Oxygen. Aluminum. 



Silicon. Calcium. 



Carbon. Magnesium. 



Sulphur. Potassium. 



Hydrogen. Sodium. 



Chlorine. Iron. 



Phosphorus. Manganese. 



Fluorine. Barium. 



Below is given a transcript of the labels used in this series. 



Oxygen. Symbol O ; atomic weight 16 : This substance is a colorless, 

 invisible gas possessing neither taste or smell. It exists in the free 

 state in the atmosphere, of which it constitutes about one fifth by bulk, 

 whilst in combination with other elements it forms nearly half the 

 weight of the solid earth and eight-ninths by weight of water. It enters 

 into combination with all the other elements, fluorine excepted, forming 

 what are known as oxides. Thus hematite and magnetite, two very 

 common ores of iron, are formed by the combination in different pro- 

 portions of oxygen and iron. 



Silicon. Symbol Si; atomic weight 28: Next to oxygen this is the 

 most abundant of the earth's constituents. It, however, never oc- 

 curs in the free state, but combined with oxygen as silica it forms more 

 than one-half of the matter of the earth's crust. 



Carbon. Symbol ; atomic weight 12 : This substance occurs in the 

 free state as graphite and the diamond, but is much more common in the 

 impure form of common coal. Uncombiued with other elements, it never 

 exists in either a liquid or gaseous form, but in combination with oxygen 

 as carbonic acid or carbon dioxide (0O 2 ) it is almost universally present 

 in the air, rain, sea and river waters. In this form also it constitutes 

 about one-fifth by weight of common limestone. It is also an abun- 

 dant constituent of many other minerals and rocks. 



Sulphur. Symbol S ; atomic weight 32 : Sulphur occurs in nature 

 in both the free and combined state. In the free state it is found in 

 volcanic regions such as Sicily, Iceland, and the Western United States. 

 Its usual form of occurrence is in combination with the metals to form 

 sulphides, or with oxygen and a metal to form sulphates. Sulphur and 

 iron combine to form iron pyrites or iron disulphide (FeS 2 ), while sul- 

 phur, oxygen, and calcium combine to form sulphate of calcium or 

 gypsum (CaSo 4 ). 



Hydrogen. Symbol H ; atomic weight 1 : Hydrogen is a colorless 

 invisible gas, without taste or smell. It occurs free in small proportions 

 in certain volcanic gases, but its most common form is in combination 

 with oxygen to form water (H 2 0), of which it forms 11.13 per cent, by 

 weight. It also occurs in combination with carbon to form the hydro- 

 carbons, such as the mineral oils (petroleum, etc.) and gases. 



