CELLS AND MANOMETER ATTACHMENTS. 21 



vents any outward lateral movement of the packing and causes the 

 latter to close up tightly on the thread of the screw. After fixing the 

 needle and the manometer tube (or rather the tube (5) which is to be 

 fused to the manometer) in their places, the cone (4) is extended by 

 means of the fusible metal (11) in order to protect the lower end of the 

 needle. Over the cone, thus extended, is slipped the rubber tube (12) 

 which is tightly wound at the lower and upper ends (13 and 14) with 

 twisted shoemakers' thread. Owing to the ease with which rubber 

 moves in the direction of smaller pressure, the whole space (14) between 

 the shoulder of the brass cone and the top of the cell must be covered and 

 rigidly supported by the thread. In practice, the winding of the upper 

 end of the rubber tube is carried so far down that one or more turns of 

 the thread are forced into the tapered neck of the cell. 



A manometer, on whose calibration, capillary depression, and final 

 verification weeks and perhaps months of labor have been bestowed, is 

 too precious an instrument to be unnecessarily exposed to danger. 

 Hence the cones are not attached in the first instance to the manometers, 

 but always to short pieces of tubing of the same kind, which are afterwards 

 fused to the manometers or cut off from them, as the occasion may arise. 



At low and moderate temperatures, the arrangement just described 

 renders very satisfactory service, and between 0° and 60° it is still in 

 use. At higher temperatures, it develops certain defects which are so 

 serious as to render its use quite impracticable. Leaks appear, due to 

 increasing difference between the expansion coefficients of brass, glass, 

 and the fusible metal; the alloy attacks the brazing or solder used in 

 attaching the hollow needle to the brass piece (Figure 9) ; the glass of the 

 manometers becoming brittle after continued use at high temperatures, 

 it is difficult to fuse them on the glass tubes which pass through the 

 cones; finally, at high temperatures, the rubber, used between the brass 

 cone and the neck of the cell, also becomes brittle and liable to crack. 



The deterioration of rubber at moderately elevated temperatures — 

 apparently due, in our case, to a resumption and continuation of the 

 vulcanizing process in the baths — has given much trouble, but we have 

 not been able wholly to dispense with its use. We are able to make tight 

 joints without it, but not, as yet, any satisfactory device for adjusting 

 pressure in the cell. 



The remainder of the manometer attachments which are here 

 described were devised for use at the higher temperatures or with elec- 

 trolytes, though they render equally good service at moderate and low 

 temperatures, and, of course, also with non-electrolytes. 



In Figure 10, the cone (a), which closes the neck of the cell (A), is 

 turned on the lower end of the brass tube (B) . At b there is a vent for 

 the escape of air and any excess of solution. The usual collar and nut 

 for fixing the manometer in the cell and for adjusting the pressure are 

 seen at c and d. The manometer (C) is held tightly in its place in the 



