ISO 



larity of weight, supposed to be the cause 

 of the identity in the size and shape of 

 molecules which cohere into the crystal- 

 line form. 



2. Iso-bryous {/3pvia, to grow). That 

 which grows equally ; a term applied by 

 some writers to dicotyledonous plants, 

 which, having two cotyledons, grow with 

 equal force on the two sides of the axis. 

 For the same reason, such plants have 

 been called iso-dynamous, from dvvafin, 

 force. See Anisobryous. 



3. Iso-chromatic (xpw/Lia> colour). Hav- 

 ing the same colours, as applied to 

 lenses. The light which is decomposed 

 by doubly refracting crystals, forms a 

 double series of coloured curves of dif- 

 ferent forms, arranged in a different order, 

 each curve in the one series having a 

 curve corresponding to it in form and 

 colour in the other. The two curves 

 which have the same tint, are called iso- 

 chromatic. 



4. Iso-chronous (xpovov, time). That 

 which occurs in equal times ; as the 

 strokes of the pulse, the vibrations or 

 oscillations of pendulums of the same 

 length, &c. Hence, the term isochron- 

 ism is applied to that property of all sys- 

 tems which are in equilibrio, by which 

 the retardation or acceleration of the 

 oscillations is not perceptibly influenced 

 by any disturbing force. 



5. Iso-clinic Lines (xXii/o), to incline). 

 Lines of equal inclination or dip ; a term 

 applied to curves which connect those 

 places in the two hemispheres where the 

 dip of the magnetic needle is equal ; they 

 surround the globe, running nearly paral- 

 lel with the magnetic equator. These 

 curves coincide in position with the iso- 

 thermal lines. See Aclinic Line. 



6. Iso-cyclous [kvkXo^, a circle). A 

 term applied to animals which are com- 

 posed of a succession of equal rings, as 

 some of the Crustacea. 



7. Iso-dynamic Lines {ivvann, power). 

 Lines of equal power; a term applied by 

 Hansteen to lines which connect those 

 places where the intensity of the terres- 

 trial magnetism has been found to be 

 equal, and which resemble in form and 

 position the isoclinic lines, though they 

 approach still more nearly to the iso- 

 thermal lines. 



8. Iso-gonic Lines {javia, an angle). 

 Lines of equal declination ; a term ap- 

 plied to lines connecting those places 

 which have an equal declination west or 

 east. Most of these lines are drawn 

 around two points near to the north and 



185 



ISO 



south poles of our planet, called the 

 magnetic poles of the earth, or, more cor- 

 rectly, magnetic poles of convergence. 



9. Iso-merism (/ut'po?, part). A term in 

 chemistry expressive of the relation ex- 

 isting becween bodies, which agree in 

 composition but differ in properties. Iso- 

 meric bodies have in general been found 

 to agree in the relative proportion of 

 their constituents only, and to differ 

 either in the aggregate number of the 

 atoms composing them, or in the mode 

 of arrangement of these atoms. Berze- 

 11 us has employed three terms to in- 

 clude the different cases of these 

 bodies, viz. isomeric, polymeric^ and meta- 

 meric. 



10. Iso-morphism (juop0>j, form). A 

 term in chemistry applied by Mitscherlich 

 to the relation in form which exists in 

 different bodies, as a general consequence 

 of similarity of composition. The law 

 at which he arrived is as follows :— -The 

 same number of atoms combined in the 

 same way produce the same crystalline 

 form ; and crystalline form is independent 

 of the chemical nature of the atoms, and 

 determined only by their number and 

 relative position. 



11. Iso-morphous {fxop(pr], form). A 

 term applied to some groups of chemical 

 substances, which, having the same 

 crystalline form, are found to enter into 

 analogous combinations, which also re- 

 spectively present the same form ; and to 

 some other groups which have been ob- 

 served to enter into similar combinations, 

 having very similar crystalline charac- 

 ters. Of these iso-morphous bodies, the 

 salts of phosphoric and arsenic acids 

 afford remarkable examples. 



12. Iso-perimetrical (nepifxeTpov, cir- 

 cumference). Having the same length 

 of perimeter, circumference, or bounding 

 line. Of all iso-perimetrical figures, that 

 is the greatest, which contains the most 

 sides or angles ; hence, the circle is the 

 most capacious of all iso-perimetrical 

 figures. 



13. Iso-poda (ttous, rrodo^, a foot). A 

 group of crustaceous animals, which have 

 seven pairs of similar unguiculate feet, 

 attached to seven moveable segments 

 behind the cephalic. A characteristic 

 example is found in the oniscus, or wood- 

 louse. 



14. Iso-pyre. A new mineral found in 

 Cornwall, imbedded in granite; it re- 

 sembles obsidian, or even some varieties 

 of iron slag. 



15. Isosceles {cKeXot, a leg). The 



