under the Influence of the Electric Discharge. 361 



employed if the heating is to be measured at points lying 

 near each other of tubes of varying width, or if the thermic 

 action is to be observed at one part of a tube and close to 

 the spectroscopic phenomena. 



The calorimetric fluid employed was turpentine. It has 

 a small specific heat when equal volumes are compared, 

 and is a good insulator, a point of special importance when 

 wide tubes are employed. The calorimeters themselves were 

 made of glass, after preliminary experiments with brass calo- 

 rimeters had shown that the fraction of the energy of the gas 

 lost by radiation was excessively small. 



The calorimeters with the discharge-tubes in them were 

 placed in a cylindrical metallic vessel with double walls, 

 being introduced either simply from above, or through an 

 opening in the bottom. In this way irregular currents of air 

 &c. were excluded, and, besides, a trustworthy correction for 

 loss and gain by radiation during the experiment was ren- 

 dered possible. 



With the horizontal tubes similar wooden boxes with 

 openings at the sides were employed. The gas to be experi- 

 mented upon was carefully dried and admitted into the com- 

 pletely exhausted tube. Only air and hydrogen were em- 

 ployed for the most part. 



As air-pump, a Topler's * pump was employed. 

 As its arrangement is not widely known, it may be briefly 

 described, with a few small changes which I have made in it. 

 A wide oval vessel A (Plate IX. fig. 5), about 15 centims. 

 high and 10 centims. wide, has glass tubes melted on to it 

 above and below. The upper one, a, is bent round immediately 

 above the globe. Its diameter is about 2 millims.; and its 

 length, reckoned from the highest point «, is somewhat more 

 than the greatest usual barometric height. Its lower end 

 plunges into a vessel L containing mercury: it is highly ad- 

 visable to cut the end of the capillary tube in L obliquely. The 

 lower tube h is about 1 centim. in diameter and 88 centims. 

 long. About 6 centims. below its junction with the bulb, a side- 

 tube B is attached, which is bent at right angles, and ascends 

 to a height of about 760 millims. above «, where (at e) it is 

 expanded into a wider cylinder for a length of about 8 cen- 

 tims., and is then bent down again; at the level of the bulb 

 it is again bent at right angles, forming a horizontal portion 

 at/, to which a vessel /3, containing phosphoric anhydride, and 

 a manometer y are attached by grinding. A tap h enables 

 the whole pump to be shut oft' from the atmosphere. To 

 facilitate the cleaning of the pump, the long tube B is made 

 * Topler, Dinyl Journ. clxiii. pp. 420-432, 1802. 



