INTRODUCTION 11 



or vapour, and at the same time a chemical change, either 

 decomposition or oxidation, takes place. In this case the body- 

 is permanently altered, and on cooling, the gas or vapour does 

 not yield the original substance. 



Of the eighty-three elements in the list on p. 3, by far 

 the larger proportion play little or no part in the ordinary 

 processes of plant and animal life. Indeed, a considerable 

 number are found in only extremely small quantities. From 

 the standpoint of the agriculturist, therefore, they possess 

 little interest. The bodies of animals and plants are mainly 

 built up of compounds of the following elements : 



Oxygen. Potassium. 



Hydrogen. Sodium. 



Carbon. Magnesium. 



Nitrogen. Iron. 



Sulphur. Chlorine 



Phosphorus. Silicon. 

 Calcium. 



A short account of these elements will now be given. 



Oxygfen is the most abundant and most important of the 

 elements. It forms about half the weight of the solid crust 

 of the earth, eight-ninths of the water, and nearly one-fourth 

 of the atmosphere. In the first and second instances the oxygen 

 is in a combined state ; in the atmosphere it exists as the free 

 element, merely mixed with the other constituents. 



Oxygen can be prepared in many ways. One of the most 

 usual laboratory methods is by the action of heat on potassium 

 chlorate, a substance having the composition expressed by 

 the formula KCIO,. The proportions of the elements present 

 in this substance are thus one atom of potassium, with a 

 relative weight of 39, one atom of chlorine, weighing 35-46, 

 and three atoms of oxygen, each weighing 16, so that in 

 39 + 35-46 + 48, t.e., 122-46 parts by weight of the salt, there 

 are 48 parts of oxygen. The final action of heat upon 



