18 



BOYS AND GIRLS IN BIOLOGY. 



means, each of them is made of only one kind of mat- 

 ter. Every substance in the world except the metals 

 and minerals, such as gold, sulphur, etc., is made up of 

 two or more of these four simple elements. Carbon 

 comes from a word which means coal, and charcoal is 

 one form of carbon — the diamond is another. Oxygen 

 means a producer of acid ; it was so called, because it 

 was supposed that it had a great power to make acids. 

 It is the most abundant of all the elements, and it is so 

 necessary to animal life, that it has been called " vital 

 air ; " more than seven million tons of it is breathed 

 every day. Hydrogen means water-producer. Water 

 is made of hydrogen and oxygen. This is the light- 

 est of all the elements, so that balloons are always filled 

 with hydrogen gas. Nitrogen means the producer of 

 nitre. It helps to make the air we breathe and the 

 protein that we feed upon. Now that we have pointed 

 out and named the different parts of each cell, and 

 found out what they are all made of, let us see if we can 

 discover the use of each part. First, the tough, woody 

 sac seems to hold and protect the soft cell-jelly or pro- 

 toplasm. The jelly, or rather the protein, which is 

 the chief part of the jelly, is the substance which makes 

 the cell alive. This is the first form of life of which 

 we know — just a simple sac filled with protein, which 

 looks like the white of an egg. All the living things 



