FIRST PRINCIPLES. 5 



animals require the substance called nitrogen to 

 form the flesh of their bodies ; and this they obtain 

 either directly from plants, or from other animals 

 which have fed on plants. Plants obtain the nitro- 

 gen which they possess from animals, or animal 

 substances in a state of putrefaction. The nitrogen, 

 when it is given off from the animal body, unites 

 with another gas, hydrogen, and the two form a 

 third gas, called ammonia, and which is well known 

 on account of its pungent odour ; it forms the basis 

 of smelling-salts, and the spirit of sal volatile. In 

 this way plants are supplied with another kind of 

 food from the animal kingdom, and the animals in 

 turn derive their food from the vegetable kingdom. 

 Such is the absolute dependence of these two great 

 kingdoms one upon the other. The interchange of 

 the four elements, carbon, hydrogen, oxygen, and 

 nitrogen, in the vegetable and animal body con- 

 stitutes the principal phenomena of life. There is 

 no existence for plants or animals but as these four 

 elements re-act upon each other ; at the same time 

 other substances exert a remarkable influence on 

 the life of both plants and animals : thus we find 

 fish living in one pond and not in another, plants 

 flourishing in one river dying directly they are 

 transported to another. The most remarkable 

 instance of this is the peculiarity of the forms of 

 plants and animals living in the sea as contrasted 

 with those living in fresh water. This does not 

 depend upon any of the general conditions to which 

 we have alluded, but simply on the substance con- 

 tained in the water ; the sea-water contains com- 

 mon salt, with a few other saline matters ; and it is 

 mainly the common salt that the varied forms of 

 animal and vegetable life which live in the sea 

 require in order to thrive and grow. That this is 

 a fact, and not an hypothesis, is proved by the arti- 



