MANUAL OF AGRICULTURE. 23 i 



liiucli used in tlie labortory for producing oxygen. The quantity 

 of this element in plant life is very minute. 



Chlorine is not found naturally free ; but when produced free, 

 it is a greenish yellow gas, pungently odorous, and most irritably 

 injurious to the mucuous membrane. For hydrogen it has a 

 strong affinity, the combination forming hydrochloric acid. Com- 

 bining with the metals, salts are produced by it, called chlorides, 

 of which the principal is chloride of sodium, or common table salt. 



Bromine, Iodine, and Fluorine all resemble chlorine in their 

 respective qualities, the four forming a detached group in 

 chemical science. Bromine and iodine are almost entirely con- 

 fined to sea-water and marine plants. Traces of Huorine are 

 found in the blood, teeth, and bones. 



Copper. — ]\Iinute traces of copper have been found in the ashes 

 of animal and vecjetable ori^janisations, but it is not considered a 

 necessary element in the economy of animal and plant life. 



The absorption, accordingly, of these inorganic or " mineral " 

 constituents by plants, is entirely effected from the soil, by means 

 of their roots. Such constituents are present in the soil in many 

 various combinations, some of them beine^ almost insoluble in 

 water. Eain-water, however, dissolves carbonic acid from the 

 atmospheric air, and water containing it in solution, can dis- 

 solve compounds insoluble in it whilst pure and simple. Car- 

 bonate and phosj)hate of lime are thus rendered available for 

 plant nutrition. It is believed that some indirect power is exer- 

 cised by plant roots themselves, in breaking up the insoluble 

 compounds in the soil. 



The different combinations in the soil affording the necessary 

 inorganic elements to plant life, and the various functions of the 

 latter to whose operation such combinations are subjected, fall to 

 be discussed in a subsequent chapter. 



Chapter III. — Of Geology. 



The student, in having his attention turned to the science of 

 geology, cannot fail to be struck with the vastness of the field 

 which is there opened out to research ; and when he encounters 

 undeniable proofs of our globe having endured through countless 

 ages ere it became fitted to receive its present species of inhabi- 

 tants, he more distinctly can realise the hopeless incomprehensi- 

 bility of the word eternity. 



WhateJ^er the source of the sixtv-three oriirinal elements, the 

 greatest physicists are of opinion that when these became united, 

 in tlie mass, resulting in an independent planet, ruled by the 

 sun's attraction, such a degree of heat must have prevailed 

 therein, as to cause such elements to exist in the gaseous state. 

 As the heat would depart by radiation into proximate space, the 

 denser compounds would tend to unite as a congeries, so that 



