Radiant Heat to Aqueous Vapour, 37 



The third column here is calculated on the assumption that the 

 absorption, within the limits of the experiment, is sensibly pro- 

 portional to the quantity of matter in the tube. The agreement 

 with observation is almost perfect. It cannot be supposed that 

 results so regular as these, agreeing so completely with those 

 obtained with small quantities of other vapours, and even with 

 small quantities of the permanent gases, can be due to the con- 

 densation of vapour on the surface of the tube. When 5 inches 

 were in the tube it had less than one-sixth of the quantity of 

 vapour necessary to saturate the space. Condensation under 

 these circumstances is not to be assumed, and more especially 

 a condensation which should produce such regular effects as those 

 above recorded. 



The subject, however, is so important that I thought it worth 

 while to make the following additional experiments : — 



C (fig. 3) is a cube of boiling water, intended for our source of 

 heat ; Y is a hollow brass cylinder, 3*5 inches in diameter and 75 

 inches in depth ; P is the thermo-electric pile, and C the compen- 

 sating cube ; S is an adjusting screen, used to regulate the amount 

 of heat falling on the posterior surface of the pile. The apparatus 

 was entirely surrounded by boards, the space within being divided 

 by tin screens into compartments which were loosely stuffed with 

 paper or horsehair. The formation of air-currents near the 

 cubes or the pile was thus prevented, and irregular motions of 

 the external air were intercepted. A roof, moreover, was bent 

 over the pile, and this was flanked by sheets of tin. The action 

 here sought I knew must be small, and hence the necessity of 

 excluding every disturbing influence. 



The cylinder Y was first filled with fragments of quartz moist- 

 ened with distilled water. A rose burner r w r as placed at the 

 bottom of the cylinder, and from it the tube t led to a bag con- 

 taining air. The bag being subjected to gentle pressure, the air 

 passed upwards amid the fragments of quartz, imbibing moisture 

 from them, and finally discharged itself in the open space between 

 the cube C and the pile. The needle moved and assumed a per- 

 manent deflection of 5 degrees, indicating that the opacity of the 

 intervening space to the rays of heat was augmented by the dis- 

 charge of the saturated air. 



The moist quartz fragments were now removed, and the vessel 

 Y was filled with fragments of the chloride of calcium. The rose 

 burner being, as before, connected with the india-rubber bag, 

 air was gently forced up among the calcium fragments and 

 discharged in front of the pile. The needle moved and assumed 

 a permanent deflection of 10 degrees, indicating that the trans- 

 parency of the space between the pile and source was augmented 

 by the presence of the dry air. By timing the discharges the 



