262 Prof. J. Dewar. [Jan. 19, 



In a similar way we obtain 



^d09 sin 6>0(sin 3 0) cos 2 ' (233). 



January 19, 1882. — From formula (226) it immediately follows 

 that — 



J;.„ ( . )=w {b. S | + |( s s)« + . . . } 



II. " Manometric Observations in the Electric Arc." By Pro- 

 fessor Dewar, M.A., F.R.S. Received January 14, 1882. 



The experiments recorded in my former paper, entitled " Studies on 

 the Electric Arc,"* together with the numerous observations made 

 conjointly with Professor Liveing on the spectrum of the arc dis- 

 charge between carbon electrodes in different gases, led me to ascer- 

 tain if the interior of the gaseous envelope of the ordinary arc showed 

 any peculiarities of pressure. Pressure might be caused by the 

 motion of the gas particles, the transit of material from pole to 

 pole, electric action, or indirectly by chemical combinations taking 

 place in the arc. As any effect due to the above causes must 

 necessarily be very small, a delicate manometer capable of measur- 

 ing easily the y-^oth of a millimetre of water pressure had to be 

 employed. The records of such an instrument in the present 

 series of experiments, are complicated by the indirect action of 

 the hot currents of air passing the poles, and the irregularities in 

 the steadiness of the arc which undoubtedly cause marked variations 

 of pressure; yet by multiplying and varying the conditions of the 

 experiments, it is possible to eliminate these secondary effects and secure 

 reliable results. The general appearance of the apparatus used is shown 

 in the diagram. A and B are two hollow carbons, similar to those I 

 formerly employed in the separation of cyanogen from the arc.f They 

 must be free from all porosity before they can be used in the experiments 

 to be detailed, and the drilled hole should not be less than 3 millims. in 

 diameter. In order to fill up minute apertures in the carbons, and render 

 them non-porous, they are placed in a porcelain tube and heated to a 

 white heat in a current of coal-gas saturated with vapour of benzole. 

 This treatment causes the deposition of a layer of dense metallic carbon 

 over the surface of the tubes which renders them capable of with- 

 standing a considerable interior pressure of air or other gas without 

 exhibiting leakage. The hollow poles are connected by means of tubing 



* "Proc. Roy. Soc," vol. 30, p. 85. 

 f " Proc. Roy. Soc," vol. 29, p. 188. 



