212 Mr. F. Guthrie on Salt Solutions 



§ 59. Chloride of Potassium. — Since KI is thus seen to com- 

 bine with 8*5 of water and KBr with 14 of water, the one a 

 fraction and the other a whole number, I spared no care in the 

 preparation and analysis of the chloride of potassium, to see 

 whether the fractional relationship did not arise from experimental 

 error. Perfectly pure K 2 C0 3 was dissolved in a slight excess of 

 HC1, evaporated to dryness, and ignited in a platinum basin. 

 The KC1 was recrystallized four times. Six ounces of the 

 saturated solution were cooled ; solid matter separated down to 

 — 10°. At this temperature the separated salt appeared as a 

 cloudiness in the liquid, a little heavier than the latter, and of a 

 granular but translucent appearance. The liquid may now be 

 cooled to —15°; but it is now a supersaturated solution of the 

 true cryohydrate — a solution, it is to be observed, of which the 

 solid crystals present are unable to determine the solidification. 

 At a little below —15°, long ice-like crystals shoot out and the 

 temperature rises to — 11 0, 4, which is the crystallizing-point of 

 the cryohydrate. From this behaviour I am disposed to admit 

 that there may be an intermediate hydrate similar to the bihy- 

 drate of NaCl. 



Of the last portion which solidified, 6*588 grms. gave 1*320 

 of anhydrous salt. Of the previous crop of crystals 7*869 

 grms. gave 1*579 of salt. These show the respective per- 

 centages 20*03 and 20*07, both of which point to the relationship 



KC1 + 16*5H 2 0. 



§ 60. The halogen salts of potassium arrange themselves ac- 

 cording to the cold required for the solidification of their cryo- 

 hydrates, and according to the molecular ratio of water, as 

 follows : — 



Number of 

 molecules of 

 Solidifying-temperature water to 1 

 of cryohydrate. of salt. 



KI . . . -23 8-5 



KBr ... -13 140 



KC1 . . . -11*4 16*5 



We find here the general rule confirmed, that among like 

 salts the lower the temperature of solidification of the cryohy- 

 drate, the smaller is the number of molecules of water which it 

 contains. 



§ 61. Iodide of Ammonium. — This salt gave me considerable 

 trouble. It is difficult to obtain in perfectly colourless crystals ; 

 and both the solid and its solution are disposed to change 

 colour, becoming brown when kept, even in the dark. Heating 

 in a water-bath with constant stirring restores the dry altered 



