204 SCIENCE FROM AN EASY CHAIR 



qualities, and that all the offspring of the same two 

 parents are not exactly alike, but often very different 

 from one another. 



Some of the simplest living things, consisting of but 

 one microscopic cell, are animals, and some are plants. 

 The essential difference between an animal and a plant 

 is shown very clearly by some of these microscopic 

 creatures. Animals feed on the flesh or " proteid " sub- 

 stances manufactured by other animals or by plants ; 

 they also feed on oils or fats, and on the sugar and starch 

 manufactured by other animals or by plants. But they 

 cannot construct these " foods " themselves from the 

 simpler stable chemical compounds called " mineral 

 bodies," which, nevertheless, contain the elements they 

 require carbon, nitrogen, hydrogen, and oxygen. Such 

 stable mineral bodies are carbonic acid, ammonia, and 

 water. In fact, ordinary " smelling salts " (which is 

 chemically carbonate of ammonia) dissolved in water, if 

 we add to it a trace of phosphates, sulphates, and 

 chlorides of potash, soda and lime, contain all the actual 

 chemical elements that an animal needs. Yet no animal 

 can be nourished by such a " mineral " soup. 



On the other hand, it is the special distinction of 

 plants of green plants, be it noted that they can feed 

 on this simple diet, and, moreover, cannot feed on any- 

 thing else. The green colouring matter which gives its 

 beautiful tint to the grass and weeds and the leaves of 

 the big trees which clothe the earth is absolutely essential 

 in this process ; so also is sunlight. The living proto- 

 plasm of the green-coloured parts of plants is crowded 

 with microscopic discs or plates of a brilliant transparent 

 green colour. The peculiar substance causing the colour 

 is called " leaf-green," or " chlorophyll." It can be dis- 

 solved out of a leaf, not by water, but by spirit or by 

 ether, and separately studied. It may be seen in solu- 



