UNITED STATES DEPARTMENT OF AGRICULTURE 



m. BULLETIN No. 1108 ^K 



Washington, D. C. T November 6, 1922 



TABLES FOR THE MICROSCOPIC IDENTIFICATION 

 OF INORGANIC SALTS. 



By William H. Fry, Scientist in Chemical Investigations, Bureau of Soils. 



CONTENTS. 



Introduction 1 



Petrographic methods 2 



Explanation of sjTnbols 4 



Page, 



Optical properties 5 



Tables 10 



INTRODUCTION. 



The identification of a salt by a qualitative chemical analysis 

 necessarily presupposes the purity of the salt, since a qualitative 

 analysis would make no distinction between an accidental impurity 

 and a constitutional molecular group of the compound. But even 

 with material of assured purity the results of the analysis would still 

 be indefinite. ^^Tiether, for example, a substance containing K, H, 

 and PO4 was a monopotassium or dipotassium phosphate would be 

 undecided. The whole question of hydrates would likewise be inde- 

 terminate. If the material qualitatively examined should be a mix- 

 ture, the difficulties of identification would be insuperable by the 

 analytical evidence alone. 



If a quantitative analysis is made, the results would be sufficiently 

 explicit m certain relatively simple cases if the material analyzed is 

 known to be a single pure substance. But in other cases — for example, 

 elemental sulphur — the analysis would give no evidence as to the 

 particular allotropic form of sulphur — -a pomt which in certain 

 problems might be of considerable importance. Where the substance 

 is a double salt, the analytical data would not differentiate between a 

 double salt and a mixture of two simple salts; and the more com- 

 plicated the molecular composition of the salt the more difficult would 

 be the interpretation of the analysis. With mixed salts any calcu- 



15:31— 22— Bull. 1108 1 



