12 INTBODUCTION. I. 



VV/H , TT 



N " 

 |H 



Amides may be regarded as being organic acids in which 

 the OH groups have been replaced by NET,. A simpler 

 amide than the above is acet amide, C a H 3 O.NH 2 , or 

 H 



H C H 



These compounds will be discussed at greater length 

 hereafter. 



Sulphur. The occurrence and properties of this element 

 are well known and need not be discussed here. In the nutri- 

 tion of plants and animals it plays a small part, but still it is 

 essential. 



It occurs in small quantity in albuminoids and, in certain 

 plants, in the form of sulphides and sulphocyanides of organic 

 bases. In animals it is particularly abundant in the hair or 

 w r ool. 



Plants probably obtain the sulphur they require from the 

 sulphates present in the soil, and in most cases from calcium 

 sulphate. It is to be noted that soluble metallic sulphides are 

 violent plant poisons, as are many other unoxidised sulphur 

 compounds (e.g., sulphocyanides, sometimes present in com- 

 mercial sulphate of ammonia). Yet it is found that certain 

 plants actually secrete sulphides and sulphocyanides of or- 

 ganic bases and owe their characteristic odour or flavour to the 

 presence of these compounds. Such is the case with mustard, 

 garlic, and many other plants. 



Phosphorus. The properties of this element are very re- 

 markable and are well known to all students of chemistry. In 

 agriculture its compounds, the various salts of phosphoric acid, 

 are of the utmost importance. 



Phosphorus is extremely widely distributed, though generally 

 in small quantities. It is present in almost every mineral and 



