“The above considerations are illustrated fairly well by the photo-micrographs shown in Figs. 68 to 71. Fig. 68 shows a section of the original untreated cocoanut shell crosswise to the long axis of the shell. In it can be seen the closely packed, thick-walled so-called ‘stone-cells’ characteristic of all hard and dense nut shells. Fig. 69 is a photograph of a similar section through the same cocoanut shell after it has been carbonized. As these photographs are all taken with vertical illumination against a dark background, the cavities, or voids, and depressions all appear black, while the charcoal itself appears white. It is clear from this photograph that much of the original grosser structure of the shell persists in the carbonized products. Figs. 70 and 71 are more highly magnified photographs of a carbonized charcoal before and after activation, respectively. As before, all the dark areas represent voids of little or no importance in the adsorptive activity of the charcoal, while the white areas represent the charcoal itself. In Fig. 70 (unactivated) the charcoal itself between the voids it seen to be relatively compact, while in Fig. 71 (activated) it is decidedly granular. This granular structure, just visible at this high magnification (1000 diameters), probably represents the grosser porous structure on which the adsorption really depends. These photographs, therefore, show how the porosity is increased by activation.” [Illustration: FIG. 71.—31-Minute Steam Activated Charcoal. Magnified 732 Diameters.] The great demand for charcoal, and the need for activating other than cocoanut charcoal led to the development of the Dressler tunnel kiln, which seemed to offer many advantages over the Dorsey type of treater. [Illustration: FIG. 72.—Sectional View of Dressler Tunnel Kiln, Adapted to Activation of Charcoal.] “The Dressler tunnel kiln is a type used in general ceramic work. The furnace consists essentially of a brick kiln about 190 ft. long, 12 ft. broad, and 9 ft. high, lined with fire brick. Charcoal is loaded in shallow, refractory trays in small tram cars, about 120 trays to the car. The cars enter the kiln through a double door and the charcoal remains in the hot zone at a temperature of about 850° C. for about 4 hrs., depending upon the nature of the material charged. Water is atomized into this kiln, and a positive pressure maintained in order to exclude entrance of air. The kiln is gas-fired and the charcoal is activated by the steam in the presence of the combustion gases.