264 



Prof. J. Dewar. 



[Jan. 19, 



with the two manometers. Two glass cylinders, ED and GF, 

 50 millims. in diameter, have each a nniform horizontal tube 

 open at both ends, 2 millims. in diameter, marked GG' and EE', 

 passing throngh the corks E and G, fitted in side apertures. 

 When fluid was added to a fixed level, these vessels constituted 

 the manometers. The tubes leading from the hollow poles have 

 been made of metal or thick india-rubber, and to prevent heating 

 of the tubes and manometer by radiation from the arc, they have 

 been carefully guarded by hollow tin screens (shown at C) through 

 which a current of water flowed continuously. The lengths of tube 

 between the manometers and poles have been varied, and in some 

 cases the tube made into a spiral form has been immersed in 

 water so as to guard against unequal heating. The little glass 

 stoppers marked Dd and Ff were convenient for the alteration of 

 the zero point by the addition or withdrawal of fluid from the 

 vessels ED and GF. In the experiments water, ether, and alcohol 

 have been used in the manometers, but the largest number of the 

 experiments have been made with ether. This fluid is most con- 

 venient from its mobility, and the only precautions to be taken are 

 to use plain cork stoppers instead of india-rubber, and to have a con- 

 siderable length of tube between the manometers and the arc. In 

 the diagram K is a millim. scale divided on glass, and H represents a 

 levelling stand on which the apparatus is placed. By careful level- 

 ling and the use of ether 1 millim. of motion of the fluid in the 

 horizontal tube may be made to correspond to about the 250th of a 

 millim. of water pressure. In the present investigation as the 

 absolute value of the pressure is not so important as the general 

 variation, I have not thought it advisable to give other than rela- 

 tive records taken with the same instrument at different times ; it 

 is quite possible, however, to get absolute values of very small pres- 

 sures by means of this instrument, and I have satisfied myself of its 

 accuracy by measuring a series of pressures of soap bubbles of 

 different sizes, which confirm the previous observations of Plateau 

 that the internal pressure varies inversely as the diameter of the 

 bubble. 



When the arc passes between two pointed carbon poles it assumes 

 two very different forms ; in one case the envelope of the intensely- 

 heated gaseous materials is well defined, almost spherical v in appear- 

 ance, surrounding the whole of the end of the positive pole, but 

 touching the negative only at a single point, without showing that 

 close adhesion to the pole which is so characteristic of the layer of gas 

 at the positive. At other times the arc is very unsteady, noisy with 

 apparent blasts of green flame-looking ejections, which generally 

 arise from the positive pole. These blasts are invariably associated 

 with a great increase of intensity in the hydrocarbon and cyanogen 



