Radiation of Heat by Gaseous Matter, 273 



to brass in reflecting power, I was unable with this source to bring 

 out with due force the vast differences existing between various 

 kinds of gaseous matter. I therefore had a copper hood con- 

 structed, and united by brazing with a tube 8 inches long, which 

 was destined to form the vacuous chamber in front of the first 

 plate of rock-salt. To heat the copper plate, a lamp formed on 

 the principle of Bunsen's burner was made use of. The gas 

 passed upwards by four hollow columns, each perforated for the 

 admission of air. The mixture of air and gas escaped from these 

 columns into a chamber shaped like the frustum of a cone, and 

 over this chamber was placed a shade of thin sheet-iron, the top 

 of which was narrowed to a slit one-eighth of an inch wide and 

 2 inches long. From this slit the mixture of gas and air issued, 

 and formed upon ignition a sheet of flame. This was caused to 

 glide along the back of the copper plate before referred to, which 

 was thereby raised to a temperature of about 270° C. To pre- 

 serve this source constant was one of the greatest difficulties of 

 the investigation; for the slightest agitation of the surrounding 

 air, or the slightest flickering of the flame itself, was sufficient to 

 disturb the steadiness of the galvanometer and to render experi- 

 ments in delicate cases impossible. The flame was surrounded 

 by screens of pasteboard, these being again encompassed by 

 towels, through the meshes of which the flame was fed; a suit- 

 able chimney produced a gentle draught and carried off the pro- 

 ducts of combustion ; the rhythmic jumping of the flame itself 

 was destroyed by screens of wire-gauze; in short, six weeks' 

 practice was required to master all the difficulties of this portion 

 of the apparatus. The "compensating cube" C, the double 

 screen H, and the thermo-electric pile P remain as before. They 

 are exposed in the figure, but during the experiments they were 

 surrounded by a close hoarding, all the chinks of which were 

 stuffed with tow, so as to protect the cube and pile from the dis- 

 turbing action of the air. To protect the anterior plate of rock- 

 salt from the heat which might have been conducted to it from 

 the source, the front chamber passed as before through a vessel 

 V in which a current of cold water, constantly renewed, was 

 caused to circulate. 



§ 2. Experiments. — On two points I wished to set my mind 

 at rest previous to starting on my vacation tour this year. These 

 were the absorption of chlorine gas and of ozone. On the 16th, 

 17th, and 18th of June I experimented on these two substances, 

 and satisfied myself that chlorine was far outstripped by many 

 colourless gases, and that ozone had a power of absorption very 

 much greater than common oxygen. 



The work was resumed on the 12th of September, and my 

 first care was to examine whether my published experiments on 



