96 On the Decomposition of Substances by Steam, 



in diameter, and having luted the openings air-tight, I heat the 

 cylinder and its contents to a high red heat. I then pass steam from 

 the boiler, through the red-hot clay-tubes, into the bottom of the 

 cylinder, and up through the charge. The heated steam, in its pas- 

 sage through the pieces of sulphate of lime, carries off the acid in the 

 state of sulphurous acid and oxygen, with sometimes a little sulphuric 

 acid mixed with it. The acid vapours pass off by the escape-tube at 

 the top of the cylinder, and I convey them by stoneware tubes into a 

 leaden chamber, in order to combine them into sulphuric acid by the 

 usual means. I take care that the heat is not raised so high at 

 first as to melt the sulphate of lime in the cylinder, but I increase 

 it towards the end of the operation, the charge becoming more in- 

 fusible when partly decomposed. I have an opening in the tube con- 

 veying off the acid vapours from the top of the cylinder, by means 

 of which I examine the vapours from time to time, and from the re- 

 lative acidity of these, ascertained by the usual tests, I. judge of the 

 progress of the operation. I regulate by a stop-cock the quantity 

 of steam passed through the charge in the cylinder, maintaining the 

 supply at that point which produces the greatest quantity of acid in 

 the vapours. When the vapours cease to contain any notable pro- 

 portion of acid, the cylinder and its contents being at a high red 

 or low white heat, I shut off the steam, withdraw the charge from 

 the cylinder by the lower opening, and put in a fresh one to be 

 treated like the first. The charge thus operated upon will be found 

 to consist chiefly of caustic lime. When I wish to obtain the acid and 

 alkaline base from the sulphate of magnesia, I first drive off by heat 

 all its water. I then introduce it, in small pieces, into a cylinder such 

 as I have before described, and operate upon it in the manner directed 

 for the sulphate of lime. But I take care to keep the heat at low 

 redness at first, to prevent the fusion of the charge, which would 

 choke up the cylinder and prevent the passage of the steam. The 

 decomposition of the sulphate of magnesia takes place at a much 

 lower temperature than that of sulphate of lime (a low red heat is 

 sufficient), and a considerable part of the acid is given off in the state 

 of sulphuric acid. When the charge has been treated as directed, 

 the residue will be found to consist chiefly of caustic magnesia. 



When I wish to decompose the sulphates of baryta and strontia, I 

 operate upon them in a reverberating furnace. This mode is less 

 advantageous for the manufacture of sulphuric acid than the use of 

 the close cylinder formerly described ; but I prefer it for the two 

 last-mentioned salts, because I consider their bases the more impor- 

 tant product of their decomposition, and the hydrates of these alka- 

 lies, and particularly that of baryta, being fusible, would have much 

 tendency to corrode the interior of the cylinder, at the heat neces- 

 sary to decompose the salts. I use a common reverberatory fur- 

 nace, with its hearth covered with a compact bed of native carbonate 

 of magnesia, 3 or 4 inches thick. Several clay steam-pipes are 



