to Calcium and Magnesium Metasilicates. 35 



When out of the melt of a readily crystallizing substance, 

 minute quantities of a second substance crystallize, they are 

 usually so completely hidden in the mass of the first crystals 

 that the process of finding them microscopically is not unlike 

 that of " finding the needle in a haystack," particularly when 

 the optical properties of the two substances are closely similar. 

 It has been found by experience that the best method for 

 detecting inhomogeneity is to immerse the powdered material 

 (finely divided by tapping the substance in a mortar) in a 

 liquid of the refractive index of the predominating substance ; 

 in this the minute particles of the second substance can be 

 seen at a glance, if its refractive index be different from that 

 of the first. For this purpose, a set of refractive liquids of 

 indices ranging from 1 *450 to 1*790 has been used, the refrac- 

 tive index of each successive liquid differing from the forego- 

 ing by "005. The refractive indices of these liquids were 

 determined directly on an Abbe-Pulfrich total refractometer and 

 their constancy checked every three months at least.* The 

 liquids are kept in small dropping bottles (30 cc capacity) 

 with ground glass stopper and ground glass cap, and the 

 refractive indices of the liquids change either very slightly or 

 not at all in three months. 



In ascertaining the different optic properties of the members 

 of this series, the following methods have been found most 

 serviceable and seem best adapted to work of this character: 

 Refractive indices were measured by use of refractive liquids 

 after the immersion method of Schroeder van der Kolk. 

 Wherever possible, especially on homogeneous preparations, 



* The following is the list of liquids used in the preparation of this set. 

 (On an average the change in refractive index of a liquid is about *001 for 



in temperature.) 



Liquids used. 



Mixtures of chloroform and carbontetrachloride. 



Mixtures of turpentine and xylol. 



Mixtures of xylol and monochlorated benzene. 



Aethyliodide. 



Mixtures of cedar oil and clove oil. 



Monochlorated benzene. 



Mixtures of cedar oil and cloye oil. 



Mixture of aethylbromide and monochlorated benzene. 



Mixtures of clove oil and cinnamic aldehyde. 



Nitro-benzol. 



Benzene monobromated. 



Mixtures of clove oil and cinnamic aldehyde. 



Mixtures of benzene mono-chlorated and a- monochlorated 



naphthaline. 

 1*640 to 1"655 Mixtures of a monochlorated naphthaline and a-mono- 



bromated naphthaline. 

 1*660 to 1'740 Mixtures of a-monobromated naphthaline and methylene 



iodide. 

 1/740 to 1*790 Mixture of methylene iodide and sulphur. 



a change of 3° G. 



Refractive Index, 



1-450 to 1-465 



1-470 to 1-495 



1-500 to 1-505 



1-510 



1-515 to 1-520 



1-525 



1-530 



1-535 



1-540 to 1-550 



1 555 



1-560 



1-565 to 1-615 



1-620 to 1-635 



