^ LECTURE II. 



Oharactcristic properties of organic substances— Relative proportions of organic elements- 

 Variable proportions of inorganic elements in plants — Forhi in which the organic ele 

 ments are taken up by plants— The atmosphere, its constitution and relations to vegetable 

 life — Nature and laws of chemical combination— Water and its relations to vegetable life 



§ 1. Characteristic properties of organic substances. 



Of the four elementary substances described in tbe former lecture, the 

 organic part of all animal and vegetable substances consists. What is 

 understood by the term organic has also been explained. 



But organic substances possess certain characters by which they are 

 distinguished from the inorganic or dead matter of tlie globe, and on 

 which their connection with the principle of life, and with the art of 

 culture, entirely depends. These characteristic properties are chiefly 

 the following : 



1°. They are all easily decomposed or destroyed by a moderately 

 high temperature. If wood or straw be heated in the air, as over the 

 flame of a candle, it becomes charred, burns, and is in a great measure 

 dissipated. So sugar and starch darken in colour when heated, black- 

 en, and take fire. The same is true of all vegetable substances. But 

 limestone, clay, and other earthy or stony matters, undergo no appar- 

 ent change in such circumstances — they are not decomposed. 



2°. When exposed to the air, especially if it be warm and moist, 

 vegetable and animal substances putrify and decay.* They decom- 

 pose of their own accord, and after a time almost entirely disappear. 

 Such is not the case with inorganic matters. If the rocks and stones 

 crumble, their particles may be washed away by the rains to a lower 

 level, but they never putrify or wholly disappear. 



3°. They consist almost entirely of two or more of the four organic 

 elements only. The mineral substances we meet with on the earth's 

 surface, and collect for our cabinets, often contain portions of many ele- 

 mentary bodies; but, with few exceptions, the organic part of all plants, 

 that which lives and grows, contains only the four simple substances 

 described in my former lecture. 



4°. They are distinguished also by this important character, that 

 ihey cannot be formed by human art. Many of the inorganic com- 

 pounds which occur in the mineral crust of the globe can be produced by 

 the chemist in his laboratory, and were any corresponding benefit likely 

 to be derived from the expenditure of time and labour, there is reason to 

 believe that, with a few exceptions, nature might be imitated in the for- 

 mation of any of her mineral productions. But in regard to organic sub- 

 stances, whether animal or vegetable, the chemist is perfectly at fault. 

 He can form neither woody fibre, npr sugar, nor starch, nor muscular 

 fibre, nor any of those substances which constitute the chief bulk of ani- 

 mals and plants, and which serve for the food of animated beings. 



* For an expleuiation of the exact nature and end of this putrefaction, see the subscquen 

 Lecture, '■^ On the decay ofanimai and vegetable substances,'' 



