28 AGRICULTURAL CHEMISTRY. 



chlorine, he will see by casting an eye at the opposite 

 colamn, is 36. In the same way he will see that the 

 atom of oxygen is 8. But the small ^ after the 

 shows that there are 5 times 8 of oxygen to 36 of 

 chlorine ; that in 76 lbs. of chloric acid are S6 lbs. of 

 chlorine and 40 lbs. of oxygen. If he look at the 

 second compound, he will see that its symbol is S 0^, 

 that is, sulphuric acid has one atom of sulphur, 16, 

 and 8 of oxygen, 8 each, making 24 ; so that 40 lbs. 

 of it would contain 16 lbs. of sulphur and 24 lbs. of 

 oxygen. On coming to the eighth he will find no 

 symbol. The reason is, that there are several oxides 

 of iron, and they could not all be represented there. 

 The two which have important relations to agriculture 

 are the protoxide and the sesquioxide. It should be 

 explained here that a pi^otoxide is one in which there 

 is but one atom of oxygen to one of the metal ; a per- 

 oxide, one in which there is much oxygen ; and a 

 sesquioxide, one in which there are three atoms of 

 oxygen to two of the metal ; that is, a protoxide im- 

 plies a low degree of oxygen ; a sesquioxide, a higher 

 degree ; a peroxide, a still higher degree ; and an 

 acid, a higher degree still. Accordingly, protoxide of 

 iron (Fe 0) implies one atom of iron, 28, to one of 

 oxygen, 8 ; and sesquioxide (Fe'' 0') implies two atoms 

 of iron, 28 each, to three of oxygen, 8 each. The 

 same is true of manganese. There is the protoxide of 

 manganese (Mn 0), and the sesquioxide (Mn'' 0'). 

 The practical relations of these two metals, particu- 

 larly of iron, will be shown in another place, and they 

 will be seen to be very important to the farmer. 



