94 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
may, in fact, be thus analysed. 1. We have a portion, almost v^^hoUy luminous, which 
went through the tube direct to the pile ; 2, a portion arrested by the first glass plate ; 
3, a smaller portion arrested by the second glass plate ; 4, we have the heat radiated by 
the first glass plate towards the second, and wholly absorbed by the latter ; 5, we have 
the heat radiated by this latter against the pile. This analysis will probably enable us 
to understand how Professor Magnus obtained an absorption of only per cent, with 
the blackened tube, and as much as 14-75 per cent, with the unblackened one. With 
the latter, the source, and the plate of glass nearest the source, send a copious flux down 
the tube to the plate at the opposite end ; the oblique rays are in great part reflected by 
the interior surface, and thus reach the plate adjacent to the pile. With the blackened 
tube this oblique radiation is entirely cut off, the rays incident on the interior surface 
being absorbed. Thus the plate of glass adjacent to the pile must be much more 
intensely heated with the unblackened tube than with the blackened one. The difier- 
ence in the amount of heat received by the pile-end plate in the respective cases is ren- 
dered very manifest by the experiments of Professor Magnus himself, for he finds that 
with the same source, twenty-six times the amount of heat transmitted by the coated 
tube is transmitted by the uncoated one. What, therefore. Professor Magnus ascribes 
to a change of quality by reflexion, would, if I am correct, be due to the higher heating 
in the case of the naked tube, and consequent greater chilling hy the cold air, of the 
plate of glass close to the pile. To this must be added the effect produced by cooling 
the distant end of the tube itself, to which heat has been communicated from the first 
glass plate by the process of conduction, and the cooling of which comes most into play 
when the tube is uncovered. 
The difference between Professor Magnus and myself as regards the action of aqueous 
vapour admits now of easy explanation. His effect being one of convection, and not of 
absorption, the quantity of vapour present in his experiments — probably not more than 
1 per cent, of the volume of the gas, certainly not 2 per cent. — vanished as a convectmg 
agent, in comparison with the air. 
It is hardly necessary to repeat these reflections with reference to the experiments of 
Dr. Feanz. The taking of the chilling of his plates for absorption, has caused him to find 
no difference of effect when he doubled the length of his tube. With a tube 450 milli- 
metres long, he finds precisely the same absorption as with a tube of 900. He finds the 
action of carbonic acid to be the same as that of air, although at atmospheric tensions 
the action of the former is 90 times that of the latter *. He finds the vapour of bro- 
mine more destructive to radiant heat than nitrous acid gas, whereas the latter is beyond 
comparison the most destructive. The heat rendered latent by the evaporation of the 
* The sensible equality of all the transparent gases and air was regarded as evident by Dr. Fraxz. “ It 
might be seen,” he writes, “from the outset that no decided difference would be observed between them” 
(p. 342). Similarly, Professor Magnus, speaking of aqueous vapour, writes, “Although it might be fore- 
seen with certainty that the small amount of aqueous vapour in the air could have no influence on the radi- 
ation,” &c. (p. 43). 
