3i8 APPENDICES. 



Optics (oTrro/icu, to see) considers the phenomena of light (and 

 vision). It is subdivided into : 



Catoptrics (KUTOTTTPOV, a mirror) treats of the reflection of light ; 



Dioptrics (SioTrrpiKos-, fit for seeing remote objects) treats of the 

 effects produced on rays of light passing through different media, 

 as through glass, water, air in other words, it treats of the refractions 

 of light ; 



Diffraction is a change which light undergoes, when, passing very 

 close to the borders of an opaque body, it forms parallel bands or 

 fringes. 



Electrology (fjXfKrpov, amber) is the science which considers the 

 phenomena and laws of electricity a subtle fluid or agent, usually 

 excited by friction. The phenomena of electricity are such as 

 attraction, repulsion, certain luminous appearances, physiological 

 effects, light, heat, mechanical violence, etc. It is the most powerful 

 agent with which we are acquainted. 



Although it has exercised the attention of men ever since Thales 

 (600 B.C.), it has become a subject of study only since Gilbert (1600 

 A.D.). But it advanced only after Franklin and Volta. The services 

 it has rendered to science and the application of science, to the 

 material improvement of the social system, since it vies with steam in 

 procuring us the benefits to be derived from rapid intercommunica- 

 tion, are too numerous and too well known to necessitate more than 

 an allusion here. 



Chemistry (\vp.ia, mixture). Physics is divided into two principal 

 branches physics and chemistry each distinct from, although in- 

 timately bound with, the other. Physics embraces the phenomena 

 and laws regulating the general properties of bodies regarded in the 

 mass ; chemistry embraces the particular phenomena modifying the 

 specific properties of matter it relates, not to masses, but to mole- 

 cules. Chemistry has, therefore, for its object the modifications that 

 all substances may undergo in their composition, alteration, decom- 

 position, in virtue of their molecular reactions. Without this new 

 order of phenomena, the most important operations of terrestrial and 

 celestial nature would be incomprehensible to us, and there is no 

 other class of phenomena so intimate and so complex. 



Theoretical chemistry was until lately divided into inorganic 

 (mineral and gaseous) and organic (plants and animals) chemistry ; 

 but the more advanced chemists refuse to admit this distinction. 



