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XLII. On a Salt that is invisible in its Mother -liquor. 

 By Charles Tomlinson, F.R.S.* 



MANY years ago Sir David Brewster*devised a simple but 

 accurate method for determining the refractive power of 

 solid fragments without the trouble of grinding and polishing 

 them. For this purpose a broken chip of the solid, so irregular 

 that no object could be seen through it, was put into a fluid of 

 the same refractive power, in which case the incident rays would 

 suffer no refraction in passing from the fluid into the solid or 

 from the solid into the fluid ; consequently objects could be seen 

 distinctly through the broken irregular chip. Thus a bit of 

 crown glass, of very irregular shape, so as to appear almost opaque, 

 became nearly invisible when put into Canada balsam, and so 

 transparent that a printed page could be easily read through it. 

 By mixing fluids of different refractive powers it is not difficult 

 to obtain a compound of the same refractive density as that of 

 the solid we wish to test. Oil of cassia mixed with oil of olives 

 in different proportions may be used for determining the refrac- 

 tive powers of all solids from 5-077 (that of oil of cassia) to 3*113 

 (that of oil of olives). 



I am not aware whether this valuable suggestion has ever been 

 adopted by persons who deal in or work up precious stones. If 

 a rough topaz or other rough stone be put into Canada balsam, 

 oil of sassafras, or other fluid of nearly the same refractive den- 

 sity, and be turned round so that the rays of light may pass 

 through in every direction, the slightest flaws or cracks are readily 

 detected. Even when the refractive density of the stone exceeds that 

 of any fluids as in the case of diamond, jasper, spinelle, ruby, and 

 some others, yet by immersing them in oil of cassia or terchloride 

 of antimony flaws and imperfections not visible and not suspected 

 start into view. Even when examined in water, flaws are more 

 perceptible than when seen in air. By this method also precious 

 stones may be distinguished from pastes. 



I do not remember any case recorded by chemists in which a 

 salt has the same refractive density as the liquor in which it is 

 formed, and is consequently invisible in it. Such a case occurred 

 to me last winter while examining the action of low temperatures 

 on supersaturated solutions, chiefly of double salts. The zinc 

 and sodic sulphates were mixed in atomic proportions, dissolved 

 in a very small quantity of water, only just enough to prevent 

 the anhydrous salt from being thrown down during the boiling; 

 the boiling solution was filtered into clean test-tubes, and was 

 protected from the action of nuclei by plugging the tubes with 



* Communicated by the Author, having been read before the Chemical 

 Section of the British Association at Liverpool, September 20, 1870. 



