CHAP. I.] THE CELL. 5 



eye even when aided by all the appliances of optical science, 

 and it is only when a number of these atoms unite into one 

 body that they become visible. Every little piece of matter 

 which we can see is built up of thousands, or rather millions, of 

 these atoms. There are many different kinds of atoms. Thus 

 we have carbon, hydrogen, oxygen, and other kinds, each having 

 its own particular weight, and probably its own particular size 

 and shape. They combine by mutual attraction, which in some 

 cases we call cohesion, and in others chemical affinity, accord- 

 ing as the atoms are of the same or of different kinds. In this 

 way an endless variety of structures may be built up. These 

 structures may be stable and solid, the myriads of atoms com- 

 posing them cohering so firmly together that it is almost impos- 

 sible to separate them ; or they may be very unstable, or complex, 

 the particles composing them being of many different kinds and 

 sizes, held together in the loosest fashion, and ready to fall apart 

 at the slightest touch. Again, we may have many structures 

 varying in stability between these two extremes. 



Protoplasm is a highly complex structure, and its chemical 

 equilibrium is so unstable that it is ready to break down at the 

 least touch. It is extremely sensitive to any outside stimulus. 

 Slight influences cause it to alter its shape, move, and exhibit 

 other phenomena; these phenomena being mainly due to the 

 inter-action of the chemical affinities of which it is composed. 



During the life of a cell its protoplasm is constantly under- 

 going changes, the chief of which may be enumerated as 

 follows : 



(1) All protoplasm coming in contact with oxygen absorbs 

 it and combines with it. Whenever this combination takes 

 place, a certain amount of the protoplasm is burned or oxidized, 

 and as a result of this oxidation heat and other kinds of energy 

 are produced, and carbonic acid evolved. 



(2) All protoplasm is able to take into itself, and eventually 

 convert into its own substance, material that is non-living ; this 

 material or food is generally supposed to be a compound chem- 

 ical substance called proteid, which is a combination of nitrogen, 

 carbon, oxj^gen, and hydrogen. In this way the protoplasm 

 may increase in amount, or in other words the cell may grow; 

 but if the amount of protoplasm does not permanently increase, 

 this is due to the fact that just as much protoplasm is being 



