874 Prof. Tyndall on the Absorption and 



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 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 com- 

 menced my researches in September with a few experiments on 

 this subject. 



Half an atmosphere of the undried air of the laboratory, ad- 

 mitted directly into the tube, cut off an amount of heat which 

 produced a deflection of 30 degrees. 



My drying-apparatus at this time consisted of a U-tube filled 

 with fragments of pumice-stone wetted with sulphuric acid. 

 Associated with this was a similar tube filled with like fragments, 

 but moistened with caustic potash solution to remove the car- 

 bonic 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 deflec- 

 tion of 26 degrees. 



This 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 arrange- 

 ment the air had entered the sulphuric-acid tube first, and passed 

 straight from the potash into the experimental tube, thus par- 

 tially 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 5 degrees. 

 Hence this experiment showed the absorption due to the moist- 

 ure and carbonic acid of the air to be more than six times as great 

 as 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 filled with frag- 

 ments of chloride of calcium, which had lain in the tube for some 

 months. The deflection produced by air thus dried was 40 de- 

 grees; 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 perplexing 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 



