Prof. Graham on Water as a Constituent of Salts. 327 



I will, when at leisure, send you some details of experiments 

 made with rough and pure copper exposed to the action of 

 muriatic acid. I am. Sir, yours, &c. 



David Mushet. 



LV. On Wafer as a Constituent of Salts. In the Case of Sul- 

 phates, By Thomas Graham, F,R.S.E., Jlndersonian 

 Professor of Chemistry and Vice-President of the Philoso- 

 phical Society of Glasgonso* 



TT may be useful to distinguish some of the functions which 

 ^ water is already admitted to discharge in the constitution 

 of hydrated salts. 



Every salt of ammonia with an oxygen acid contains an 

 atom of water, and cannot exist without it. The state of com- 

 bination of the water is peculiar, and has been represented by 

 supposing that the elements of ammonia unite with the hydro- 

 gen of the water, and form a new compound radical, to which 

 the name ammonium is given, while the oxygen of the water 

 unites with this radical, and produces oxide of ammonium. 

 Hence nitrate of ammonia, in which there exist the elements 

 of one atom of nitric acid, of ammonia, and of water, is viewed 

 as anhydrous nitrate of the oxide of ammonium, and corre- 

 sponds with nitre or the nitrate of the oxide of potassium. 

 But it is not the object of this paper to discuss particularly 

 the state of water in the ammoniacal salts. 



We have it often in the crystals of salts, united by a feeble 

 affinity, and known under the name of water of crystallization. 

 The number of atoms of water with which some salts unite, in 

 crystallizing from a state of solution, is affected by tempera- 

 ture, and other slight causes. This water is commonly viewed 

 as a constituent of salts which is not essential, owing to the 

 facility with which it may in general be expelled by heat, and 

 also to the circumstance that many salts usually hydrated, are 

 likewise capable of existing in a crystalline state without 

 water. 



In the hydrates of the caustic alkalies and of the earths, 

 water is retained by a strong affinity, and is generally sup- 

 posed to be united, like an acid, to the alkali or earth. In 

 such hydrates, water discharges an acid function. 



In the case of hydrates of the acids, the portion of water 

 which is found to be inseparable by heat, or to be very 

 strongly retained, has generally been presumed to be in the 

 place of a base to the acid, although little attention has been 



* From the Transactions of the Royal Society of Edinburgh, vol. xiii. 

 Part 1. recently published : revised by the Author. 



