40 Frederick Guthrie on Salt Solutions 



bath only just to complete dryness. If the substance be further 

 heated, minute iridescent crystals begin to appear at that part 

 of the residue where the basin is in contact with the bath. As 

 soon as these began to appear the further heating was stopped. 



grams. Nitrate. Per cent. 



6*6889 gave 0*5693 or 8-64 

 6-7616 „ 0-5844 „ 8*51 



The water-worth is accordingly 72*83. As a cryogen the 

 temperature of — 4°*5 is obtained. 



§ 177. Oxalate of Potassium. — The temperature at which 

 the cryohydrate formed was found to be —6° 3. On evapo- 

 rating 3*2862 grams in a platinum capsule, igniting, and 

 quickly cooling on a metal slab, 0*5790 of the carbonate was 

 obtained ( = 17*62 per cent.). 



In another analysis, 3*1500 grams, on evaporation to dry- 

 ness on a water-bath, gave 0*6684 oxalate (21*22 per cent.). 

 This gave on ignition 0*5531 of carbonate (17*56 per cent.), 

 which by treatment with hydrochloric acid gave 0*5958 of 

 chloride of potassium (18*88 per cent.). The percentage of 

 carbonate agrees closely with that of the first analysis. The 

 percentage of chloride differs only by 0*07 from that required 

 on the conversion of 17*56 of carbonate. Hence it would 

 seem that with due care oxalate of potassium may safely be 

 estimated as carbonate, notwithstanding the hygroscopic cha- 

 racter of the latter salt. It also appears (contrary to state- 

 ments sometimes made) that oxalate of potassium does not 

 retain any of its water of crystallization when its solution is 

 evaporated to dryness at 100° C. ; for the 17*56 per cent, of 

 carbonate obtained is equivalent to 21*12 per cent, of oxalate, 

 which only differs by 0*1 per cent, from 21*22, which was 

 found. The water-worth (deduced from the chloride of potas- 

 sium) is 17*3. 



The temperature of the cryogen is — 6°*2. 



§ 178. Fluoride of Sodium. — This salt, like several other 

 salts of sodium, is troublesome when attacked from the side of 

 saturated solutions, because there appears to be a subcryohy- 

 drate. This salt also exhibits the very rare property of being 

 almost equally soluble at all temperatures between 100° and 

 — 3°*9. On cooling a hot saturated solution to —2°, nothing- 

 separates. Between —2° and —4° an ice-like body is formed. 

 The true cryohydrate appears to be formed at — 5°*63; for this 

 temperature is preseryed till the whole is solid and opaque. 

 Fluoride of sodium, as a cryogen, gives — 3°*2. The cryohy- 

 drate has not been further examined. 



§ 179. Cyanide of Mercury. — A concentrated solution gave 



