32 HOW CROPS GROW. 



traction, and cannot ordinarily exist in the free state. 

 On this view each particular kind of matter or chemical 

 substance owes its indiTiduality either to the special kinds 

 or to the numbers of the atoms it consists of. Atoms 

 may be defined as the smallest quantities of matter which 

 can exist in chemical combination and the smallest of 

 which we have any knowledge or conception. 



Atomic Weight of Elements. — On the hypothesis 

 that chemical union takes place between atoms of the 

 elements, the simplest numbers expressing the propor- 

 tions by weight* in which the elements combine, are ap- 

 propriately termed atomic weights. These numbers are 

 only relative, and since hydrogen is the element which 

 unites in the smallest proportion by weight, it is assumed 

 as the standard unit. Prom the results of a great 

 number of the most exact experiments, chemists have 

 generally agreed upon the atomic weights given in the 

 subjoined table for the elements already mentioned or 

 described. 



Symbols. — For convenience in representing chemical 

 changes, the first letter (or letters) of the Latin name 

 of the element is employed instead of the name itself, and 

 is termed its symbol. 



TABLE OF ATOMIC WEIGHTS AND SYMBOLS OF ELEMEXTS.t 



* Unless otherwise stated, parts or proportions by weight are always 

 to be understood. 



t Now, chemists receive as the true atomic weights double the num- 

 bers that were formerly employed, those of hydrogen, chlorine and a 

 few others excepted. The atomic weights here given are mostly whole 

 numbers. The actual atomic weights, as experimentally determined, 

 differ from the above by small fractions, which may be neglected. 



