TECHNICAL CHEMISTRY. 331 



difference of level that there is a column of heated gases 

 several feet in height immediately over the fire-bars, and 

 the upward movement of this heated and rarefied air 

 receives a check in going round the muffle, so that the 

 ordinary state of things is reversed, and the pressure in 

 the flue round the muffle is greater than that in the interior 

 of the muffle, thus altogether preventing any chance of 

 escape of the acid vapours from the muffle to the chimney, 

 though allowing some of the gaseous products of combus- 

 tion to pass through the unavoidable cracks into the interior 

 of the muffle, but in such small quantity that they do not 

 interfere at all with the working of the process. I have 

 little doubt that before long either this or some similar 

 salt-cake furnace will come into general use. 



We owe another valuable and quite recent improvement 

 in the salt-cake process to Messrs. Jones and Walsh of 

 Middlesborough. This consists in a mechanical arrangement 

 by which all hand labour is dispensed with, and by which 

 the whole operation, from the mixing of the materials to the 

 production of the finished dry salt-cake, is carried on in one 

 large pan. 



A third proposal made by Messrs. Cammack and Walker 

 seems to me to be based upon a more scientific view of the 

 decomposition than any of the former plans. When large 

 masses of salt and sulphuric acid are brought together, the 

 reaction as you see when I pour this bottle full of acid on 

 to the salt contained in this flask is at first very violent, 

 and torrents of hydrochloric acid gas are evolved. The 

 action however soon moderates, as you notice. During the 

 first twenty minutes the main quantity of acid has come off 

 from the salt-cake pan, and during the remaining three 

 hours needed to complete the reaction, probably only a 

 small quantity of gas enters the condensers. So that for 

 twenty minutes the condensers have more work than they can 

 properly do, whilst after that time they are underworked. 

 Messrs. Cammack and Walker's plan seeks to obviate these 

 inconveniences of all the old processes by sending in at one 

 end of the heated space in which the reaction occurs a con- 

 stant stream of salt and acid mixed in the right proportions, 

 and drawing off at the other end the finished salt-cake. Thus 

 a never increasing and never decreasing stream of hydro- 

 chloric acid gas is sent into the condensers, and a constant 

 supply of the solid product is furnished. In this way the 



