142 C. W.Cook — Crystal Form of AlkaU Halides. 



Art. X. — The Crystal Form of Some of the Hydrated Halide 

 Salts of the Alkali Metals ; by C. W. Cook. 



Some years since, an attempt was made to prepare, for the 

 purpose of crystallographic study, a complete suite of crystals 

 of the alkali halides. The proper conditions for crystal growth 

 were not obtained, however, so that the crystals which resulted 

 were imperfect and for the most part not suited for study. In 

 a number of instances, hydrous instead of anhydrous com- 

 pounds were formed. With the exception of the lithium salt 

 of hydrobromic acid, these compounds upon exposure to the 

 air lost their water of crystallization so rapidly that the crystals 

 became opaque and their surfaces so dull that accurate meas- 

 urements could not be made. In two cases, those of sodium 

 bromide and potassium fluoride, the loss of water was less 

 rapid than for most of the compounds, so that a few measure- 

 ments were possible. While these measurements are not com- 

 plete and probably not accurate, the results are given in hopes 

 that they may be of assistance to future investigators. 



Hydrated Potassium Fluoride (KF, 2H 2 ? ). 



This compound was prepared by treating a solution of 

 potassium carbonate placed in a lead dish with hydrofluoric 

 acid and allowing it to crystallize through slow evaporation at 

 room temperature (cir. 20° C). 



The crystals which were obtained were about 7 to 8 mm in 

 length with smooth well-developed faces. On exposure to the 

 air, they rapidly became opaque and the faces assumed a pitted 

 appearance. The substance crystallizes in the prismatic class 

 of the monoclinic system and the crystals under discussion 

 possessed a decidedly prismatic habit. The elements of crys- 

 tallization are: 



a : b : c = 1-0565 : 1 : 0-70608 (3 = 109° 15', 



which are based upon the following observations : 



a : m = (100) : (110) = 44° 52' 

 a : c = (100) : (001) = 70° 25' 

 c : g = (001) : (011) = 33° 38' 



Five forms were observed as follows : 



ajioo;, b{o\o\, cjooi}, mjiioj, g{on\. 



Definite signals were obtained from all of the faces except 

 those of the basal pinacoid, so that the observations except for 

 the value of /9 are probably fairly accurate. The value of /9 is 



