38 Chemical and Physical Notes 



it is no longer believed to be a definite hydrate in the 

 chemical sense of the term, still it is a mixture of NaCl 

 and H 2 O in a definite and constant proportion with a con- 

 stant melting-point, and simulates a chemical compound so 

 successfully that it is well entitled to retain its name of 

 cryohydrate, all the more as it is convenient to have a special 

 name to designate these mixtures. The cryohydrate of 

 chloride of sodium is an intimate mixture of 29*6 grms. of 

 chloride of sodium with 100 grms. of ice. It melts at the 

 constant temperature 2i'85 C, and when melted it will, 

 if cooled, solidify again at the same temperature. Below 

 this temperature chloride of sodium and ice are indifferent 

 to each other ; above this temperature they melt each other, 

 and between this temperature and o C. chloride of sodium 

 is a deliquescent salt. 



Again, we see from de Coppet's table that the cryohydrate 

 of chloride of ammonium (NH 4 C1) consists of 22-9 grms. of 

 the salt and 100 grms. of ice, and this mixture can exist either 

 in the liquid or the solid state at - I5'8C. In the case of 

 chloride of sodium it was indicated that by making a solution 

 saturated at ordinary temperatures (+15 C.) we obtained 

 a solution of the concentration of the cryohydrate, which 

 is a solution saturated at the cryohydric temperature 

 ( 2i'85 C.). It is a remarkable property of common salt 

 that its solubility in water is almost entirely unaffected by 

 change of temperature. The law holds almost universally 

 that salts are more soluble in warm than in cold water. 

 This is the case with chloride of ammonium. At a baro- 

 metric pressure of 742 mm., when steam is blown through 

 the salt it forms a boiling mixture of salt and saturated 

 solution having the constant temperature of 1 1 3'8 C., the 

 temperature of boiling distilled water under the same pressure 

 being 99*33 C. This boiling saturated solution contains 

 787 grms. of chloride of ammonium to 100 grms. of water, 

 and on cooling to the ordinary temperature of 15 C. the 

 amount of the salt remaining in solution is only 357 grms. to 

 100 grms. of water, the difference, 43 grms., has separated 

 out as crystals. If the solution, saturated at +15 C. and 



