Chemical Equilibrium of Solids. 11 



is also described in the paper last cited. I need only add that the 

 ends of the brazed copper bath are provided with screw stuffing- 

 boxes of the ordinary kind, in which either asbestos packing 

 or a hollow cylinder of cork may be forced home by the gland. 

 Vapour-baths must be removed hot. They then slide off the 

 smooth steel piezometer easily, and without jarring the 

 tube. 



4. In the compressor, the arrangement adopted is that of 

 insulating the piezometer-tube from the barrel- and com- 

 pression-appurtenances. Inasmuch as in some of the experi- 

 ments the resistances to be measured amount to several 

 hundred megohms, the piezometer-insulation must be perfect 

 to at least several thousand megohms. How this was gradu- 

 ally accomplished will be explained in §§ 10 et seq. Gutta- 

 percha-covered wire and raised hard-rubber commutators are 

 essential. Care must be taken to insulate the water-jackets 

 which cool the ends of the piezometer, and to allow the efflux 

 water to drop into a cistern discontinuously. The burners 

 which heat the vapour-bath must be insulated to prevent con- 

 duction through the flame, &c. At high temperatures, where 

 the resistance of the glass will have decreased several hundred 

 to one, many of these precautions are superfluous. They are 

 always necessary when oils are examined. 



Fig. 1. — Tube for measuring the Pressure-increments 

 of the Electric Resistance of Glass. 



5. The original apparatus for measuring the resistance of 

 compressed glass is shown in figures 1 and 2, of which the 

 latter is a diagram. Fig. 1 consists essentially of two coaxial 

 glass tubes, the larger completely surrounding the smaller. 

 The diameter of the larger (thick- walled) tube is *4 to '5 

 centim. ; that of the inner (thin- walled) tube is '2 centim. 

 They are joined along the ring a a. Two separate compart- 

 ments are thus formed, the inner of which is filled with pure 

 mercury, and the outer with sodium amalgam, to a sufficient 

 length to completely envelop the inner tube. The end of 

 the large tube contains hydrocarbon oil, both to protect the 

 amalgam against moisture or air, when the tube is not in place, 



