CELLS AND MANOMETER ATTACHMENTS. 9 



the cell, when burned, exhibits upon its exterior surface a series of 

 spirally arranged elevations or depressions, as if the shrinkage of the 

 clay in baking had not been entirely uniform. Considerable difficulty 

 was experienced at first in securing the correct temper for the grooved 

 disks, which, of course, should be equal to, but not much higher than, 

 that of the steel balls which separate them. If the disks are insuffi- 

 ciently tempered, they are badly lacerated by the balls. On the other 

 hand, if they are made too hard, they frequently crack under the great 

 pressure to which the clay is subjected. 



In order that the diameter of the clay cylinders may be varied, the 

 barrel of the press (Figure lA)i& made quite wide (2.5 inches internally) 

 and is provided with a series of steel "sleeves" of various smaller bores — 

 which may be inserted. Each sleeve requires, of course, its own set of 

 disks (Figure 1 A 5, 6, 7, and 8, and Figure 1 B). The length of the cyl- 

 inder is regulated by the number and thickness of the disks (5), which 

 are placed in the bottom of the press before introducing the clay. 



The two clays, prepared as previously described, mingle readily in all 

 proportions, giving products which, when baked, are uniform in respect 

 to texture and strength. It was found that all the requirements of the 

 situation are best met by mixing them in about equal proportions by 

 weight. The process of mixing is as follows: (1) Equal weights of the 

 air-dried and pulverized clays are mingled and repeatedly sifted; (2) 

 the mixture is churned with water for several hours, after which (3) it 

 is bolted — without unnecessary interruption of the churning process- 

 through Nos. 14 and 16 bolting-cloth; (4) the material is allowed to 

 subside, and the supernatant water is removed by means of a siphon ; 

 (5) the major portion of the large excess of water still remaining with 

 the clay is removed by draining upon a filter of bolting-cloth resting 

 upon one of paper, or by the "endosmose" method already described ; (6) 

 finally the material is extensively kneaded and mixed upon a plate- 

 glass surface until, through evaporation of the water, the "putty" has 

 attained the consistency which experience has shown to be best suited 

 to pressing. 



The putty, which must never be touched without first covering the 

 hands with rubber gloves, is "tamped" down in the press with a steel 

 plunger which has been cleansed with ether. 



The device for compressing the clay is shown in Figure 2, without the 

 framework which holds the various parts in their places. The press 

 (1) containing the clay is secured between two flat bars of steel, a por- 

 tion (2) of one of which is seen in the figure. The lower end of the 

 vertical shaft (3) is square in form, like the upper end of the plunger of 

 the press (Figure 1 A), and the collar (4), which joins the two, has a 

 square hole of the same diameter passing through it. A portion of two 

 of the timbers of the framework is shown in the figure (5 and 6). 

 Through these, the shaft (3) slides freely up and down, except so far as 

 its motion is limited by the set collar (7). The large wooden drum (8) 



