704 



plaliniim wires about 1 inm. thick were originally sealed into a 

 glass tube. It appeared, however, that such sealing places were no 

 longer to be trusted after the tube had been compressed to e. g. 

 2500 atra. Sometimes they lasted some time longer, often however 

 they already came forth cracked from the pressure apparatus, and 

 in any case the reliability was exceedingly slight. Amagat too 

 complains of the great fragility of his tubes. An investigation under- 

 taken specially for this end showed that the cau.se of the phenomenon 

 will be found in the ditïerence of compressibility between enamel 

 glass and platinum, in consequence of which the connection between 

 the wire and the glass is lost at high pressures. 



This gave an indication of the way in which improvement was 

 to be expected. If only the platinum wires are taken exceedingly 

 thin, the change of volume cannot be so great that detaching is to 

 be feared. Glass tubes in which capillary wires o*' 0.0356 mm. of 

 Hartmann and Brain were sealed, appeared really not to lose anything 

 of their strength, not even when they had been kept at 3000 atm. 

 for a long time. It is, however, not possible to seal in these wires 

 in such a way that the mercury forms contact against a loose point 

 of them ; it is self-evident that they are too limp for this. This 

 difficulty can be overcome by not letting a bit of wire stick out in 

 the lube, but by sealing in the wii-e at both its extremities. The 

 whole tube is therefore made as follows. A thick-walled capillary 

 tube of Jena enamel glass is blown out to small reservoirs in 15 

 or 20 places. At the top there is a somewhat larger reservoir, above 

 which the tube is drawn out to a very narrow capillary. Under the 

 said widening there are a number of very small reser\'oirs, which 

 pass into reservoirs that become gradually larger, to distribute the 

 points as uniformly as possible over the isotherm that is to be 

 determined. Now the tube is cut through at the places between the 

 reservoirs, which have kept their normal thickness of wall; a 

 platinum wire of the said strength is laid between the two ends, so 

 that the wire projects outside on either side, and then the glass is 

 fused together again. In this the wire is bent downward in a point 

 to get a sharper contact with the mercury. Then the projecting ends 

 are connected with a spiral of the same platinum wire, wdiich is 

 attached to the tube by means of "zapon"-lac and gelatin. (See B'ig. 4. 

 For clearness' sake the wire is drawn beside the tube. P'ig. 5 gives 

 the real position). If the mercury is Ciuite at the bottom of the 

 tube, the resistance between the two leads is the total amount 

 of the platinum spiral, e. g. 150 Ohms. As soon, however, as the 

 mercurj' has risen to the second contact B, the resistance AB e. g. 



