ELEMENTS ATOMIC WEIGHTS MOLECULES, ETC. 3 



plant food comes from the decomposition of organic sub- 

 stances in the soil. The other six elements necessary for 

 plant growth are required in comparatively small amounts. 

 With the exception of phosphorus, soils usually contain 

 an abundant supply of these " ash elements." 



In the diagram, Fig. 1, is shown the relative propor- 

 tion of eight of the most abundant of the elements as found 

 in the atmosphere, all waters, and the solid parts of the 

 earth's crust which have been examined. It will be noticed 

 that the seventy-five elements not mentioned, altogether, 

 comprise but 2.7 per cent of the earth's constituents. With 

 the exception of a comparatively small quantity of oxygen 

 existing in a free condition in the air, this figure represents 

 the percentage of the elements as they exist in compounds. 



3. Composition of Matter. Matter may be divided and 

 subdivided till definite parts called molecules are reached. 

 Finally the molecule, by chemical means, may be separated 

 into invisible particles called atoms. The atom may be 

 defined as the extremely minute particle of matter that 

 enters as a unit into chemical combinations with other 

 atoms. A molecule is the smallest part of matter that can 

 exist by itself. An atom does not remain free or uncom- 

 bined; it unites either with other atoms of the same kind 

 to form a molecule of an element or it combines with atoms 

 of a different kind and thus produces a compound. 



4. Atomic Weights. Atoms combine chemically accord- 

 ing to definite proportion by weight. The smallest amount 

 of hydrogen that will enter into chemical reaction, i.e., 

 the hydrogen atom, is less by weight than the atom of 

 any other element. For this reason hydrogen may be 

 taken as a convenient unit of comparison and its smallest 

 combining weight, which is the weight of its atom, may 

 be assumed to be one. The gases hydrogen and chlorine, 

 when mixed and exposed to light, will form a new substance, 

 hydrogen chloride, which in its water solution is called 

 hydrochloric acid. When one part by weight' of hydrogen 



