GLOSSARY 298 



Heat of Ionization. The heat absorbed or set free 

 by the electrolytic dissociation of a compound in 

 water. See Electrolytic Dissociation. 



Heat of Solution. The amount of heat absorbed 

 or set free when a given quantity of a substance is 

 dissolved in a solvent. The amount depends on the 

 substance, the solvent, and quantity. 



Heliotropic. Characterized by or relating to heli- 

 otropism. See Tropism. 



Hertzian Waves. "Wireless" waves. Very long 

 electric waves. Electric waves lie below the region 

 of the heat waves. The wave-length of the longest 

 Hertzian waves is over a mile. They are named for 

 their discoverer, Heinrich Hertz. 



Histologic. Concerned with the minute structure 

 of tissues. See Tissue. 



Hydrolysis. A reaction between water and an- 

 other compound (as salts of weak bases or weak 

 acids) in which the second compound suffers chemi- 

 cal decomposition and the water splits up into H 

 and OH, one of the decomposition products com- 

 bining with H and the other with OH. 



Hysteresis, Chem. A term used to denote a lag 

 or retardation in passing into a stable condition. 



Hysteresis. Physics. "The tendency of a magnetic 

 substance to persist in any state of magnetization." 



Isomerism. The condition of two or more (or- 

 ganic, or carbon) compounds of having identical 

 molecular formula (i.e., the same chemical compo- 

 sition) but exhibiting different physical properties, 

 and in many cases different chemical proper- 

 ties. Compounds thus characterized are termed 

 isomers or isomerides. Organic chemistry shows a 

 great many cases of isomerism. Several special 

 types of isomerism are recognized; as, stereoisom- 

 erism (isomerism due to the different arrangement 



