268 Frederick Guthrie on Salt Solutions 



containing 292 molecules of water. Accordingly 



Chromate, K 2 Cr0 4 . -1L 18-8 



Bichromate, K 2 Cr 2 7 . - 1 292*0 



§ 84. Ammonium Alum. — This body was examined on account 

 of the large percentage of water which is held by the ordinary 

 hydrated salt. The cryohydrate solidifies at — 0°*2. The part 

 last to solidify was examined ; of this, 4*38 15 grms. contained 

 0*2060 grm. of anhydrous alum. This shows 4*7 per cent. Of 

 the immediately preceding crop of crystals, 5*2400 grms. gave 

 0*2220 of anhydrous alum, or 42 per cent. The first of these 

 determinations indicates the relationship 



AI 2 NH 4 2S0 4 + 261*4H 2 0. 



§ 85. Per 'chloride of Mercury. — A saturated solution of cor- 

 rosive sublimate solidifies at — 0°*2. The ultimate and penulti- 

 mate portions were examined. Of the former, 6*0640 grms. 

 gave 0*197, or 3*24 per cent. ; of the latter, 4*977 grms. gave 

 0*164, or 3*29 per cent. The first indicates the formula 



HgCl 9 + 450H a O. 



§ 86. Oxalate of Ammonium. — This solidifies as a cryohydrate 

 at — o, 2. Of the last portion to solidify, 4*340 grms. contained 

 0*125, or 2*8 per cent, of the anhydrous salt. This agrees with 

 the relation 



NH 4 C0 2 +2391H 2 0. 



§ 87. Carbonate of Sodium. — After igniting the carbonate so 

 as to decompose any bicarbonate, the saturated salt solidifies as 

 a cryohydrate at —2°. Of the final portion, 6*4090 grms. con- 

 tained 0*383 of Na 2 C0 3 . This shows 5*97 per cent., or the 

 relation 



Na 2 C0 3 + 92*75 H 2 0. 



§ 88. The following Table shows at a glance the relation be- 

 tween the lowest attainable temperature when the salt is mixed 

 with ice, the temperature of the solidification of the cryohydrate, 

 the water-worth or aquavalent. Column 1 shows the salt em- 

 ployed, and the degree of hydration when associated with water 

 of crystallization. Column 2 shows the temperature obtained 

 when the salt is mixed with ice. Column 3 shows the tempe- 

 perature at which the cryohydrate separates. In column 4 are 

 shown the number of molecules of water associated with one 

 molecule of the salt in the cryohydrate. It is here called 

 " water-worth." In column 5 are the percentages of the anhy- 

 drous salt which the final portions of the cryohydrates contained, 



