GLOSSARY 285 



by its presence is capable of influencing the reactions 

 between certain other substances, while remaining 

 chemically unchanged itself. 



Cathode Rays. A stream of electrons emitted 

 from the negative pole (cathode) of a vacuum tube. 



Centrosome. An organ of the cell that is found at 

 the center of greatest activity in the processes of 

 cell-division. 



Chemical Action. The process in which atoms 

 unite to form molecules or molecules are broken up 

 into atoms. Generally, in chemical processes mole- 

 cules are both formed and decomposed. Chemical 

 action always is accompanied with changes in energy. 



Chemical Affinity, "the force which binds the atoms 

 together in their combinations as molecules," is 

 measured in terms of energy. The quantitative re- 

 lations between the free energy change (change in 

 the energy which can be obtained in the form of 

 work) and the total energy change of a reaction in 

 condensed systems is expressed in Nernsfs Heat 

 Theorem. The amount of heat liberated or absorbed 

 in a chemical reaction between given quantities of 

 substances is termed the heat of reaction. The heat 

 of formation is "the amount of heat liberated or ab- 

 sorbed in the formation of one gram-molecule of a 

 compound from its elements." Hess's Law states the 

 amount is the same whether it is formed in one or 

 more reactions. The heat of combustion is "the total 

 heat liberated by the complete oxidation of a given 

 quantity of an element or compound." And so on. 

 (See Heat of Solution.) The idea that chemical af- 

 finity is electrical in nature, has been entertained 

 since the days of Sir Humphry Davy (Bakerian 

 Lecture, 1806) and of Berzelius (1812). Faraday 

 said that the forces of chemical affinity and electric- 

 ity are the same. 



