EADIATION OF HEAT BY GASEOUS IMATTEE. 
65 
numerous applications of the apparatus. In all the experiments the tube remained 
perfectly transparent throughout, and, on pumping, the needle in each case returned 
accui’ately to zero. 
§3. 
In my last paper I brought the fact somewhat prominently forward that the elementary 
bodies which I had then examined were far less hostile to the passage of the longer 
undulations than the compound ones ; and I founded at the time certain theoretic con- 
siderations on this fact*. I was desirous this year to extend the experiments to one or 
two of the coloured gases and vapours, and on the 20th of September resumed my expe- 
riments on chlorine. This gas is itself highly coloured, and of a specific gravity of 
2-45 ; one of its compounds, hydrochloric acid, is quite transparent, and of specific 
gravity of only 1‘26. Does the act of combination with hydrogen which renders 
chlorine gas more transparent to light render it also more transparent to heat 1 Chlorine 
prepared from hydrochloric acid and peroxide of manganese, and dried by passing it 
through sulphuric acid, was admitted into the tube till it depressed the mercury gauge 
21 inches ; the absorption of the gas was expressed by the number 44. 
Hydrochloric acid was admitted till it depressed the gauge 19 inches; the absorption 
was 68. This experiment indicates that transparency to light and opacity to heat 
accompany the same act of chemical imion. 
The following results were afterwards obtained. I may remark that a subsidiary 
gauge was used, so as to prevent the destructive gases from entering the air-pump. 
Absorption. 
Chlorine 15 inches .... 32 
Chlorine 14 inches .... 30 
Chlorine 14 inches . ... 30 
Hydrochloric acid 14 inches . 47 
Chlorine again 30 
Hydrochloric acid . . . . 56 
In all cases the effect of the compound gas was found to exceed that of the elementary 
one ; so that the chemical change which renders chlorine more transparent to light renders 
it mcn'e opake to obscure heat. 
Great care is requhed in experiments on hydrochloric acid, and great care was 
bestowed on the above. Previously to the introduction of the gas the experimental tube 
was filled with perfectly dry ah, so as to leave a perfectly dry residue on exhaustion. 
The gas was allowed to stream through the drying-tube until all traces of air were 
expelled; then a joint was suddenly broken, and the retort was connected with the 
experimental tube. The gas thus passed dhectly from the retort through the drying 
apparatus into the vacuum. It was difficult to avoid sending in with the gas a few 
particles of moisture ; but these, if such existed, appeared to be dissipated by the dynamic 
* Philosopbical Transactions, 1861. 
MDCCCLXII. 
K 
