( G32 ) 



per and then again thi-ough ferrous sulphate and sodium hydroxide, 

 because otherwise traces of nitric oxide might bo left and this blocks 

 the cocks (this gas is recognised at once by a strong smell of higher 

 oxides of nitrogen when it mixes with the air). In order to remove 

 traces of this oxide, I have sometimes added to the gas a quantity of 

 oxygen as nearly as possible equivalent to tlie NO contained therein 

 and have then passed it through sodium hydroxide. The gas is col- 

 lected and i)r()visionally kei)t in galvanised iron vessels holding 1 M'. 

 From these it is driven out later b}' ^vater lieated by a steam jet 

 and after passing through sodium hydroxide and sul[)huric acid it is 

 forced into a small gasholder floating on oil and holding 500 L. 

 By means of the auxiliary compressor AC lubricated with glycerine 

 (see PI. VI and for details Comm. 54 Sept. '99) and the mercury 

 compressor H</C (see PI. VI and for details Comm. 54) the gas is 

 forced over into a metal cylinder of 18 liters capacity after passing 

 through the drying tul)es />,, J).^ filled with caustic soda in tiie form 

 of sticks. 



Plate VI 8ho^^■s the scheme of the entire circulation with the 

 cryostal Cr, into \vliich the liquid nitrogen is admitted at a and where 

 it e\'aporates under ordinary or reduced pressure at the desired tem- 

 1 (oral ure. The whole arrangement has been used in the comparison 

 of the platinum resistance thermometer witli the hydrogen thermo- 

 meter, which has been mentioned in 111. The apparatus themselves 

 are drawn diagrammalically l)ut in their true proportions, while the con- 

 nections are entirely schematic. A detailed representation of the cryostat 

 with the auxiliary apparatus appertaining to it for uniform and constant 

 temperatures will l)e found on plate I where tlie same letters have 

 been used. On the other hand plate VI may l)e considered as a sup- 

 plement to plate I. Nothing is wanting for a complete representation 

 of the circulation except the gasholder and the vacuum vessel of 

 5 M\ (comp. ^ 5 for its use) which are too large to be repre- 

 sented on the same drawing as the parts given. There is an 

 insignificant difference in the coupling of the leads between plate I 

 and plate VI, for on plate VI Ei'h. 1' indicates the connection of the 

 compression side of a Burckhardt — Weiss vacuumpump Bu Vac, 

 described below into which the exhaust E.vh. 2 terminates, to an 

 exhaustpump (which may also be AC of the circulation). Moreover 

 next to the lead from Y^ to F35 we have drawn what must be sub- 

 stituted for it in comparison with the arrangement on plate IV. 



RM is the cylinder where the nitrogen has been compressed by 

 means of AC and HgC through the drying tubes D^ and i)j, while 

 Gaz indicates the 500 liter gas holder floating on oil. The nitrogen may 



