214 Mr. F. Guthrie on Salt Solutions 



faculties with potassium • namely, with both, iodine gives a 

 lower-solidifying cryohydrate than bromine, and bromine lower 

 than chlorine ; and we have here also confirmation of the rule 

 that in the same series the aquavalents diminish with the tem- 

 perature of solidification. 



§ 64. Bromide of Sodium. — To complete this series I take 

 now the halides of sodium. The bromide of sodium sepa- 

 rates from a saturated solution as a cryohydrate at —24° C. 

 Taking the last and the next to the last portions, 7*5010 grms. 

 of the former gave 3*1000 of anhydrous bromide ; of the latter, 

 8*3160 grms. gave 3*4605 of NaBr. The corresponding per- 

 centages are 41*33 and 41*61, the first of which indicates 8*12 

 and the second 8*08 molecules of water to 1 of the salt. 



NaBr + 8*12H 2 0. 



§ 65. Iodide of Sodium. — A saturated solution of iodide of 

 sodium presents in the most remarkable manner the phenome- 

 non of supersaturation. It may be cooled to —22°, freely ex- 

 posed to the air, and shaken without solidifying. On being 

 placed in contact with solid carbonic acid and ether, it solidifies, 

 and its temperature instantly rises to —15°. The solidification 

 once started by this extreme cold, the temperature remains con- 

 stant at —15°. Fragments of the cryohydrate so formed in- 

 duce solidification in other portions subjected to the ice-salt 

 cryogeu. The last two portions were analyzed. Of the very 

 last, 6*4000 grms. gave 3*8050 of Nal, or 59*45 per cent. Of 

 the next to the last, 10*2450 grms. gave 6*0845, or 59*39 per 

 cent. The molecular ratio of the cryohydrate is accordingly 



Nal + 5*82 H 2 0. 



Correlating the iodide and bromide of sodium with the chlo- 

 ride, we have, therefore, 



Solidifying-temperature 

 of cryohydrate. 



Number of 



molecules of 



water to 1 



of salt. 



Nal . . -15 

 NaBr . . -24 

 NaCl . . -23 



5*82 

 8*12 

 10*5 



§ 66. It appears that the iodide of sodium forms the first ex- 

 ception we have yet met with to the general rule ; for while its 

 solidifying-point is above that of its companions, it attaches to 

 itself a less number of water-molecules. In the Table below the 

 whole nine combinations are shown ; and they are there arranged 

 according to the number of molecules of attached water, or in 

 what might be called their " aquavalents " if this expression were 



