TECHNICAL CHEMISTRY. 333 



chloric acid contained in this volume has been absorbed by 

 the water, and we now only need to precipitate with silver 

 nitrate and estimate the chloride of silver in order to learn 

 how much acid there is in each cubic foot of chimney gases. 

 This, multiplied by the number of cubic feet passing up 

 the chimney in any given time, as ascertained by the 

 very ingenious anemometer devised by Mr. A. E. Fletcher, 

 one of the inspectors under the Act, gives the total amount 

 of escaping hydrochloric acid in that particular time. 



Before leaving the salt-cake process, I must refer to a 

 new and very interesting mode of preparing this substance 

 by a process differing from that of Leblanc, which from the 

 name of the inventor is termed the Hargreaves process. The 

 object of this is to manufacture salt cake directly from salt, 

 sulphur dioxide (sulphurous acid) and water, so as altogether 

 to do away with the manufacture of sulphuric acid. It 

 depends upon the principle, that although sulphurous acid 

 cannot by itself decompose salt, it is able to do so in the 

 presence of air, or rather of oxygen, and of vapour of water, 

 if time enough be allowed. 



In order to effect this decomposition, a series of 

 large kilns or stoves are built of brick, so arranged that 

 each kiln can be put into communication with its neighbour, 

 and each heated by a fire. Each kiln is then filled with 

 dried and porous salt (NaCl), and the gases from the pyrites 

 burners led directly into these kilns one after the other. 

 By careful attention to temperature and to the quantity of 

 air and steam admitted with the sulphur dioxide, it is possible 

 in this way to decompose the salt as completely into 

 sulphate of soda (salt-cake) as when heated by the old process 

 with sulphuric acid. The chemical reaction, I need scarcely 

 say, is just the same in both cases, thus : 



2 NaCl + S0 2 + + H 2 O = Na 2 S0 4 + 2HC1. 



Hydrochloric acid gas is given off from the Hargreaves 

 kilns as from the ordinary salt-cake pan or roaster, and 

 must in like manner be passed into the condensers. 



2. The Black-ash Process. I must now ask you to accom- 

 pany me through the second portion of Leblanc's system, 

 the so-called Black ash process. 



The theory of this process is a simple one, so far as 

 the chief products are concerned, bat it is complicated 



