EADIATION OF HEAT BY OASBOHS MATTEE. 
61 
The brass tube, then, which stretched from S to S' in the figure is now replaced by one 
of glass, 2 feet 9 inches long, and 2 ‘4 inches in diameter. The source of heat in my 
last-published inquiry was the cube of hot water C ; but glass being far inferior 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 constructed, and united by brazing with a tube 8 inches long, which was destined 
to form the vacuous chamber in front of the fii’st 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 frus- 
tum 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 preserve this source constant was 
one of the greatest difiiculties of the investigation; for the slightest agitation of the 
surrounding afr, or the slightest flickering of the flame itself, was sufficient to disturb the 
steadiness of the galvanometer and to render experiments 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 suitable chimney produced a 
gentle draught and carried ofF the products of combustion ; the rhythmic jumping of 
the flame itself was destroyed by screens of wu’e-gauze ; in short, six weeks’ practice was 
required to master all the difiiculties of this portion of the apparatus. The “ compen- 
sating 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 stufied with tow, so as to protect the cube 
and pile from the disturbing 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 moist and dry air stood the test of repetition. 
Professor Magnus had experimented on dry air and on air saturated with moisture, and 
found that the presence of the moisture had no influence on the absorption. I, on the 
