Properties and Activities of Living Protoplasm 79 



Molecules are too small to be visible with the highest magnification of 

 an ordinary microscope which uses Hght rays, because the smallest par- 

 ticle visible with such a light (optical) microscope must have a diameter 

 of 150 millimicrons (1 millimicron is one-millionth of a millimeter and 

 is abbreviated m/x). The largest molecules probably have a diameter 

 of approximately only 1 millimicron. The term millimicron is used 

 here instead of millimeter or centimeter because the latter are too 

 large for measuring and recording such minute objects. However, an 

 electron microscope which uses beams of electrons instead of beams of 

 light can be used to photograph the larger molecules even though they 

 do not produce an image which can be seen with the eye. 



HYDROGEN (h) 



cakbon(c) 



(o 



NITROGEN (n) 

 7 



OXYGEN (O) 

 8 



MAGNESIUM (Wq) 



?HOSPHORUS(P) 

 15 



SULFUR (s) 



POTASSIUM (k) 

 19 



Fig. 25. — Diagrams representing the structure of the. atoms of certain of the 

 elements which may be present in protoplasm. The atomic number, which is the 

 same as the number of nuclear protons, is given below each name. The symbols 

 in parentheses follow each name. P, proton (+ electrical charge) ; N, neutron 

 (electrically neutral); small circle with a dash, an electron (- electrical charge). 

 The inner circle represents the nucleus of the atom; the outer rings represent 

 one or more orbits ("shells") of the electrons. 



Molecules are constantly in rapid motion, moving about in their inter- 

 molecular spaces. Their speed depends upon certain conditions and 

 varies over wide ranges, but an average speed is thought to be approxi- 

 mately 2 million million times their own diameter per second (about 

 twenty miles per minute) in such a substance as a gas. No matter how 

 sparsely distributed, molecules cannot travel so fast and so far without 

 colliding with other molecules also in motion. This energy of molecular 

 movement is an example of kinetic energy (Gr. kinein, to move). The 

 intermolecular space between molecules in a gas is greater than the space 



