Chemical Science, 22$ 



form. After three months (exposure, it contained, on July 18th, 

 7.4 proportions of water instead of 12, its full number. Re- 

 duced to powder and exposed in thin layers on paper to air till 

 the 26th of the same month, it gave 6.5 proportions. Again ex- 

 posed during a hot and dry period until the 31st of July, it gave 

 only 5.65 proportions. Then being left exposed until the 21st of 

 October, the quantity of water had increased to 7.2 proportions. 

 Phosphate of soda, which had been calcined, acquired half a pro- 

 portion of water by being exposed to air for five days. 



Carbonate of soda has the same habitudes as the phosphate ; it 

 becomes opaque, and loses much water without changing its form, 

 but it never becomes anhydrous. 



It results from these observations that some salts lose all their 

 water of crystallization by exposure to air, whilst others retain va- 

 riable quantities according to the hygrometric state of the atmos- 

 phere. I do not pretend to say that definite quantities of water 

 may not be retained, but only that, in the phosphate and carbonate 

 of soda, the affinity which connects a certain proportion of water 

 the seventh, for instance, with the salt, is very little different from 

 that which combines the proportion immediately above or beneath 

 it. — Annales de Chimie, xxxvi. 334. 



6. Anhydrous Crystals of Sulphate of Soda. — If a drop of a solu- 

 tion of sulphate of soda be placed upon a glass plate and allowed to 

 evaporate spontaneously, it will leave crystals which may be distin^ 

 guished by their form and ultimate efflorescence, as being the salt 

 in question. Most of the potash and soda salts may be distin- 

 guished as to their base by such an experiment. They are easily 

 converted into sulphates by a drop or two of sulphuric acid and 

 ignition, and then, being dissolved and tried as above, will yield 

 crystals which may be known by their forms, and more espe- 

 cially by their efflorescence if of soda, and their unchangeable state 

 if of potash. This test is, however, liable in certain circumstances 

 to uncertainty, arising from a curious cause. If the drop of solution 

 on the glass be allowed to evaporate at common temperatures, then 

 the efflorescence takes place and the distinction is so far perfect ; 

 but if the glass plate with the drop upon it ha placed upon a warm 

 part of a sand bath or hot iron plate, or in any other situation of 

 a certain temperature, considerably beneath the boiling point of 

 the solution, the crystals which are lefl upon evaporation of the 

 fluid are smaller in quantity, more similar in appearance to sulphate 

 of potash, and finally do not effloresce. Upon examining the 

 cause of this difference, I found they were anhydrous; conse- 

 quently incapable of efflorescing, and indeed exactly of the same 

 nature as the crystals obtained by Dr. Thompson from certain hot 

 saturated leys*. 



Hence it would appear that a mere difference in the temperature 



» Quarterly Journal, xxii. 399. op Ann. Phil. N. S. xx. 40 1. 



