Properties of Pure Substances : Nitrogen, 23 



similar to it, but of only 1 centim. diameter, and having a 

 finer constriction of thermometer-tube in the bent part. This 

 tube was not so good as the last, as it was not gas-free, and 

 a small correction amounting to half a millimetre or so had 

 usually to be applied on this account. This correction was 

 determined several times, and a table of readings and correc- 

 tions made out. There is no uncertainty as to the readings 

 beyond about *05 millim., a quantity wholly insignificant for 

 our purpose. This gauge lasted till 1892, when I finally 

 got tired of applying corrections, and made a new and better 

 gauge on the same lines and of the same dimensions. This 

 is still in use. 



Tube Pressure-Gauge. — A glance at the Plate will show 

 that this gauge was similar to the one used by Professor 

 Thomson and myself in 1885, with the exception that it was 

 of glass throughout. A tube of about 2 centim. diameter is 

 bent into a U-shape, the bend being of tube about *5 millim. 

 in diameter. The limbs are about 5 centim. long. The mer- 

 cury is introduced so as to stand about halfway up the wide 

 tubes. The upper ends of the limbs are connected to each 

 other, so that the gauge is really a " ring " space, and the 

 space above the mercury communicates with the experimental 

 tube by a filamentary glass tube of from one to two metres in 

 length. When it is desired to observe the change of pressure 

 in the spark-tube, the connexion between the space over the 

 free surface of the mercury in the two limbs is interrupted by 

 fusing a previously narrowed neck into a solid mass. This of 

 course allows the pressure in the closed limb to retain its 

 initial value, whatever that may be; while the pressure in the 

 open limb varies with the pressure in the tube. 



The effect of the metre-long filamentary tube is to hinder 

 the diffusion of mercury vapour into the sparking-tube; and 

 this in one or two experiments was also attended to by in- 

 cluding a tube filled with clean silver foil between the mercury 

 and filamentary tube. The filamentary tube, however, never 

 let enough mercury-vapour in to give a mercury-spectrum at 

 the lowest pressure at which I worked. Of course the mer- 

 cury in the gauge was carefully boiled several times in order 

 to cause any considerable quantity of gas that might have 

 been condensed to be given off before the experiment began. 

 These gauges, of which large numbers were made at one time 

 and another, were very satisfactory, and when observed with 

 proper precautions by means of the cathetometer, would cer- 

 tainly betray a difference of pressure of about *05 millim., all 

 sources of uncertainty being taken into account. 



