CONSTRUCTION OF RETORTS. 69 



expensive. Originally they were built in brickwork singly, and heated by 

 flues passing beneath and over them, without any guard, except in some in- 

 stances that of an iron saddle. They were afterwards placed in pairs, then 

 in a greater number ; but nevertheless, until the guard of fire-tile was used, 

 the wear and tear was enormous. 



The great obstacle to working more than two retorts to one furnace evi- 

 dently arose from the difficulty of conducting the heat by means of flues 

 around the series of retorts in such a manner that it should act with equal 

 force on all. Different workmen constructed these flues in different ways : 

 in short, the forms varied in every possible manner, and still with the same 

 result. Mr. Rackhouse, in the year 1815, constructed a set of retorts on the 

 oven-plan now generally adopted, and to that gentleman solely are we in- 

 debted for the contrivance. The fuel required for heating the retorts, when 

 set without guards on the previous plan, was less by nearly ten per cent, than 

 that required for the same purpose on the oven-plan ; but the greater duration 

 of the retorts much more than compensated for the additional fuel. I do not 

 mean to assert that the first construction by Mr. Rackhouse was so perfect as 

 the arrangements adopted in later years ; every invention is gradually im- 

 proved upon, but his principle has not been altered ; the later improvements 

 are only modifications, pointed out by experience, and rendered necessary by 

 circumstances which the greatest foresight could not anticipate. 



The oven represented in Plate I. is one of the latest arrangements. The 

 heat from the furnace passes through the square openings M at each side, 

 and is thus equally divided along the whole length of the retorts ; from be- 

 tween the walls N it rises between the fire-tiles at the outer sides of the 

 lower retorts. The flame is not suffered to impinge upon any part, but is 

 equally distributed throughout the oven, and consequently the retorts work 

 and " burn out" evenly. The lower retorts, which would otherwise be ex- 

 posed to a more direct heat, are carefully guarded by fire-tiles, which at the 

 same time prevent the bottoms from bulging. The openings O at the top of 

 the main arch act more in the manner of safety-valves than flues, serving to 

 regulate the final exit of the heated air, and, being distributed along the outer 

 length, they do not draw the flame to one part. 



The whole interior of the oven, as well as those parts in contact with the 

 flame, must be constructed of Newcastle fire-bricks set in Newcastle clay. 

 The main arch, six feet in span and half a brick in thickness, is formed of 



