62 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
contrary, had previously found, and stated, that dry air had only a small fraction of the 
absorptive energy of the same air when even partially saturated. I commenced my 
researches in September with a few experiments on this subject. 
Half an atmosphere of the undried air of the laboratory admitted directly into the 
tube cut ofi’ an amount of heat which produced a deflection of 30 degrees. 
My drying apparatus at this time consisted of a U-tube fllled with fragments of pumice- 
stone wetted with sulphuric acid. Associated with this was a similar tube fllled with 
like fragments, but moistened with caustic potash solution, to remove the carbonic acid 
of the air. 
The air of the laboratory passed through both these tubes in succession, till a tension 
of 15 inches was attained, gave a deflection of 26 degrees. 
Tliis result surprised me, showing, as it seemed to do, a very close agreement between 
dry and moist air. On examining the drying- tubes, however, I found that by a mistake 
of arrangement the air had entered the sulphuric acid tube first, and passed straight 
from the potash into the experimental tube ; thus partially reloading itself with moisture 
after it had been dried. 
The air was now sent through both tubes, commencing with the potash — the deflection 
fell instantly to less than 6 degrees. Hence this experiment showed the absorption 
due to the moisture and carbonic acid of the air to be more than six times greater than 
that of the atmosphere itself. It will presently be seen that the difference here stated 
falls far short of the truth. 
The potash and sulphuric acid were now abandoned, and the air was dried by passing 
it through a U-tube fllled with fragments of chloride of calcium, which had lain in the 
tube for some months. The deflection produced by air thus dried was 40 degrees ; that 
is to say, 10 degrees more than that produced by the undried air. 
This result, and many others of a similar nature, were due to the imperfection of the 
chloride of calcium. I think chemists ought to be very cautious in the use of this 
substance as a drying agent. When pure and newly fused it may answer for this pur- 
pose, but when old it yields an impalpable powder, which proved in the highest degree 
pei-plexing to me in my first experiments. It is generally found, I believe, that a drying- 
tube of sulphuric acid gains more in weight than one of chloride of calcium, and from 
this it has been inferred that the quantity of moisture taken up by the former is greater 
than that taken up by the latter. The difference, however, may really be due to the 
mechanical carrying away of a portion of the chloride by the current of air. 
On the 13th of September these experiments were resumed. The dry air then gave 
a deflection of less than 2 degrees ; the air direct from the laboratory caused, in one 
experiment, the needle to move from 20 degrees on one side of zero to 28 on the other. 
In a second experiment the undried air caused the needle to move from 18° on one side 
of zero to 32° on the other. 
Experiments made on the 17th entirely corroborated this result. Three successive 
trials made the action of the undried air of the laboratory 29°, 31°, and 30° respectively; 
