68 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
events, if any further advance should be made in the purification of the gases, it will 
certainly only tend to augment the enormous differences exhibited in the above Table. 
Ammonia, of the tension mentioned, stands highest in the above list as regards absorp- 
tive energy. I believe that a length of less than 3 feet of this gas, which to the vision 
is as transparent within the tube as the vacuum itself, is perfectly hlacJc to the rays 
emanating from the source here made use of. When the gas was in the tube, the inter- 
position of a double metallic screen between the pile and source augmented the deflec- 
tion very slightly. But I shall show further on that the ammonia in this experiment 
could not exhibit the full energy of its absorption, and that the length indicated is in 
all probability absolutely impervious to the heat issuing from our source. 
It would he a mere affectation of accuracy to try to deal with smaller quantities of the 
first four substances mentioned in the Table than those with which I have here operated. 
Still, if such small quantities could be directly measured, the action of air, oxygen, 
hydrogen, and nitrogen, in comparison with that of the other substances at the same 
tension, would doubtless be greatly reduced. With the energetic gases the rays are 
most copiously struck down by the quantities which first enter the tube, the quantities 
which enter last producing in many cases an infinitesimal effect. Now I have shown 
in my last paper that for very small absorptions the effect is sensibly proportional to 
the quantity of gas present, and this would seem to justify us in assuming that for 
1 mch of tension the absorption of air, oxygen, nitrogen, and hydrogen would be ^th 
of the absorption at 30 inches. With all the other gases I have measured directly the 
absorption of a quantity possessing in each case a single inch of tension. Assuming the 
proportionality just referred to, and again calling the effect of air unity (the unit, how- 
ever, being only -^th of that in the last Table), the following are the relative absorp- 
tions : — 
Table II. 
Air I 
Oxygen I 
Nitrogen I 
Hydrogen I 
Chlorine 60 
Bromine 160 
Hydrobromic acid . . . 1005 
Carbonic oxide .... 750 
Nitric oxide 1590 
Nitrous oxide I860 
Sulphide of hydrogen . . 2100 
Ammonia 7260 
Olefiant gas 7950 
Sulphurous acid . . . . 8800 
Here we have the extraordinaiy result that for tensions of 1 inch of mercury the 
