100 On the Decomposition of Substances by Steam, 



steam thrown into the salt retort, and by that means, decrease the 

 supply of salt vapour driven into the cylinder. When the escaping 

 vapours contain but little salt, and a large quantity of acid, I consider 

 the operation as proceeding favourably, and I always endeavour to 

 regulate the quantities of steam passed through the two pipes, and 

 by that means, the proportions of salt vapour and steam thrown into 

 the cylinder, so as to produce this effect. 



When the escaping vapours contain a large quantity of salt and 

 steam and but little acid, the cylinder and its contents being at a 

 high red heat, I consider that the decomposition of the salt is no 

 longer effected in the cylinder, and I then shut off both currents of 

 steam, withdraw the charge by the lower door, and replace it by 

 fresh alumina. The withdrawn charge is then lixiviated with hot 

 water, and the solution of aluminate of potash or soda thus obtained 

 is treated with carbonic acid, as before described. 



The lining of the cylinder should be examined occasionally, and 

 kept in repair, so that the fire-clay may not be corroded by the 

 alkali. Provided the charge of alumina in the cylinder is readily 

 and equally permeable to the current of steam and salt vapour, the 

 smaller the pieces of which it consists, and the greater the surface 

 they expose to the current, the more rapidly will the decomposition 

 of the muriate proceed. The steam used need not be of a higher 

 boiler-pressure than will suffice to secure its passage through the 

 charge in the cylinder. The subphosphate of alumina may be sub- 

 stituted for the alumina, in the processes for the decomposition of 

 the sulphate and muriates of potash and soda, and its action is even 

 more powerful, but its first cost is greater. Although to aid the 

 decomposition of the sulphates and muriates of potash and soda by 

 steam at a high temperature, the use of either alumina or its sub- 

 phosphate is preferred as the combining substance, yet there are a 

 great number of substances which also possess the requisite proper- 

 ties, but act with various degrees of energy. Thus many salts which 

 contain already a certain proportion of base, will yet, when exposed 

 in contact with the sulphates and muriates of potash and soda, at a 

 high heat, to the action of steam, form a combination with the pot- 

 ash or soda, decomposable, when cold, by water, or water and car- 

 bonic acid. The subphosphates of lime, baryta, and strontia, and 

 the subsilicates of lime, baryta, and strontia, will, under these cir- 

 cumstances, combine with the alkali and yield it to the action of 

 water alone when cold. The sulphates of baryta and strontia, al- 

 though themselves decomposable by the action of steam at high tem- 

 peratures, are still capable of thus aiding in the decomposition of the 

 sulphates and muriates of potash and soda, and yield the alkali by 

 the action of water. The neutral phosphates, and neutral silicates 

 of potash and soda, when thus treated, form basic salts which are 

 soluble in water and decomposable by carbonic acid. 



The alkalies, lime, and magnesia, will also thus combine with a 



