SUBSTANCES PRODUCED BY PLANTS. 61 



by uniting with water, or with the gases hydrogen and oxygen 

 which compose it, is capable of producing woody fibre, starch, 

 sugar, and the various oils and acids. The same elements 

 also, by uniting in diflerent proportions with nitrogen, derived 

 from the ammonia of the atmosphere, and with a small pro- 

 portion of sulphur and phosphorus taken up from the mineral 

 matters of the soil, form a class of compounds approaching 

 closely in their composition to the substances of which the 

 bodies of animals are composed. These compounds are de- 

 signed for food — are, in fact, the ready-formed materials of 

 blood and flesh. It is surely well calculated to excite admi- 

 ration when we reflect how, out of half-a-dozen of elements, 

 such a variety of important compounds is produced. If you 

 take a piece of the muscle of an animal — a piece of mutton 

 chop, for example — and examine it, you will find that it con- 

 sists chiefly of a fibrous substance. If you pour water upon 

 it, you can render it quite white — ^you will wash away the blood 

 upon which its red colour depends. Upon examining it you 

 will also find that a portion oifat is mixed up with it. If you 

 dry the flesh and burn it, you will find that, like the plant, 

 it consists of two parts — a part which disappears into the 

 air, and an incombustible ash which remains. This ash, 

 when examined by the chemist, is found to contain the very 

 same substances that we have described as composing the 

 incombustible part of plants. I have already stated that 

 the gluten, the albumen, and casein of the vegetable world 

 are almost identical with the fibre of muscle, and that the 

 fatty matters which exist in the seeds of plants (78) contain 

 the same elements, united in nearly the same proportions, as 

 the fat with which the human body is supplied to facilitate the 

 movements of our joints and muscles. Thus, in the interior 

 of the plant. Nature prepares a store of materials, which, like 

 the ready-formed wheels and screws that the watchmaker 

 has merely to put in their proper places, in constructing or 

 repairing a watch, are capable, when taken into the stomach, 

 of being at once selected and applied to -build up the frame 

 and covering of the body. The soil of the field and the car- 

 bonic acid, the watery vapour, and the ammonia of the air, 

 contam the elements of flesh and blood ; but these elements 

 must undergo certain changes before they can serve us for 

 food. The office of plants is to effect these changes. They 

 are the agents incessantly at work extracting from the at- 



