134 ANNUAL OF SCIENTIFIC DISCOVERT. 



but only intermediately, through the f:ict that a definite chemical 

 constitution produced a definite cr3'stalline form ; that is, a definite 

 arrangement of the material molecules in space. The so-called 

 cases of dimorphism were considered by the author in connection 

 with his views, and he denied the possibility of dimorphism in the 

 ordinary sense of the term. It might be accounted for in every 

 known case by the differences in chemical constitution produced 

 by impurities. For instance, arragonite is never pure carbonate 

 of calcium, but is always associated with variable quantities of the 

 carbonates of lead, strontium, and manganese. 



INDEX OF REFRACTION. 



At a recent meeting of the Roj'^al Institution , Dr. Gladstone stated 

 that the indices of refraction of liquids are lowered by increase of 

 temperature ; he showed, by means of a glass prism filled with 

 oil of nutmeg, that the more the oil was heated the less could it 

 bend a ray of light from the electric lamp out of its primary direc- 

 tion. This law holds good with all liquids. Solids, on the con- 

 trary, have their power of refraction very slightly increased as 

 they are heated, with the exception of crown glass, which is not 

 influenced in its optical properties by temperature, and fluor spar, 

 which, like a liquid, has its power of refraction diminished by an 

 increase of heat. 



The specific refractive energy — or the refractive index, minus 

 unit}", divided by the density — is common to the solid, liquid, 

 and gaseous conditions of bodies.- A general law discovered by 

 him is, that the refraction equivalent of any substance is the sum 

 of the refraction equivalents of its constituents. The specific re- 

 fractive energy multiplied by the atomic weight of the substance 

 gives its refraction equivalent. He expressed the opinion that the 

 double refraction of Iceland spar may be caused by the crystal 

 being in a state of unequal tension in different directions, so as to 

 permit light to pass more freely in one direction than in another. 



NEW POLARIZING APPARATUS. 



This apparatus, made by M. Hem pel, from a suggestion by M. 

 Plucker, of Bonn, similar in the majority of its parts to one 

 already known, is made up as follows: 1, A black glass, usu- 

 ally horizontal, but capable of being inclined, ujDon which the 

 incident ray is polarized rectilineally by reflection ; 2; a disc or 

 ring, to carry and maintain in the path of the reflected polarized 

 ray the transparent plate which is to exhibit chromatic polariza- 

 tion ; 3, a convex lens which renders the ray leaving the trans- 

 parent plate parallel or convergent; 4, (the addition which gives 

 to the instrument a perfection scarcely expected), a parallel glass 

 silvered on the exterior surface, and fixed in a support at such an 

 inclination that it leads to the vertical, the doubly reflected and 



