PART III. 



ORGANIC CHEMISTRY. 



GENERAL OBSERVATIONS. 



Organic substances, whether derived from the vegetable or 

 animal kingdom, are chiefly remarkable for the complexity of their 

 composition, and for the limited number of their elements. Only 

 four elements are considered essential to the composition of organic 

 matter, viz. carbon, oxygen, hydrogen, and nitrogen, — though others 

 are occasionally met with, as sulphur, phosphorus, chlorine, sodium, 

 &c. ; altogether they do not amount to more than fifteen. There 

 appears, however, to be no limit to the number of definite compounds 

 which may be produced out of merely the four above-named essen- 

 tial elements, simply by a difference in their proportions and mode 

 of arrangement. 



In consequence of the complexity of organic bodies, they are 

 generally very instable, being prone to decomposition whenever the 

 restraining force is removed. The products of such decomposition are 

 water, carbonic acid and ammonia; and if sulphur be present, sul- 

 phuretted hydrogen. 



As a general rule, the more complex is the constitution of an 

 organic body, the more liable is it to decomposition ; but this ten- 

 dency is much lessened if the elements are in such proportions as 

 completely to saturate each other; thus in sugar, starch, and lignin, 

 the proportions of oxygen and hydrogen are exactly sufficient to 

 saturate each other, and to form water. 



Vegetable organic matter is usually ternary in its composition, 

 and is not so prone to decomposition as animal organic matter, 

 which is generally quarternary. Both are invariably decomposed 

 by heat. 



Isomeric bodies are frequently met with among organic substances. 

 By this term is meant bodies having the same chemical composition, 

 but possessing very different properties ; starch, sugar and gum are 

 examples. Isomerism is believed to depend upon a differe7it arrange- 

 ment in the constituent atoms of a body, — their number remaining 

 the same. 



