OXIDES AND OXYGEN ACIDS OF SULPHUR 269 



wanted other methods of driving off the water, such as the cas- 

 cade system or the Gaillard tower, must be employed. 



The cascade system consists of a series of small silica or silicon- 

 iron basins set over an inclined flue, and so placed that each basin 

 delivers by a spout into the one below. The flue is heated by 

 gas or coke firing at the lower end, and dilute acid is fed continu- 

 ously into the basin at the top end. As the acid passes from 

 basin to basin, it meets hotter and hotter conditions and becomes 

 more and more concentiated. 



The Gaillard plant consists essentially of a large hollow tower 

 built of acid-resisting stone, and filled with small fragments of 

 similar material. Dilute acid is sprayed in at the top, and meets 

 hot furnace gases injected in at the bottom. Most of the water 

 contained in the acid is carried off by these gases, and concen- 

 trated acid collects at the bottom of the tower. 



A more convenient method of obtaining very concentrated acid, 

 which avoids the difficulties of evaporation entirely, is to add to 

 the chamber acid the requisite quantity of oleum, prepared by the 

 contact process already described. Commercial sulphuric acid, 

 oil of vitriol, has a specific gravity 1.83-1.84, and contains about 

 93.5 per cent H 2 SO 4 . 



Physical Properties of Sulphuric Acid. The pure acid is a 

 colorkss, oily liquid of sp. gr. 1.84, which freezes to a solid at 10. 

 It mixes with water in all proportions, and much heat (heat of 

 solution, see p. 244) is given out when it dissolves. It boils at 330, 

 but the vapor is largely decomposed into free water and sulphur 

 trioxide, which recombine when it cools. 



Chemical Properties. 1. The acid is more stable than sul- 

 phurous acid, but has a slight tendency to lose SO 3 even at ordi- 

 temperatures and as already noted, decomposes largely at 

 the boiling-point. 



2. In aqueous solution, sulphuric acid is much more active 



