340 Mr. T. Tate's Experimental Researches on the 



Here the near coincidence of the results given in the second 

 and third columns, shows that the weight of vapour diffused 

 through the different columns of air in the same time is very 

 nearly inversely as the depths of these columns. In this re- 

 spect the law of diffusion of vapour through air is similar to 

 the diffusion of a liquid through the pores of an absorbent. 



It has been shown, the atmospheric column being constant 

 and the temperature variable, that the amount of vapour dif- 

 fused is proportional to the tension of the vapour at that particular 

 temperature ; hence we conclude that the amount of vapour dif- 

 fused varies directly as the tension of the vapour at the given 

 temperature, and inversely as the depth of the column of air 

 through which the vapour has to pass. 



9. The time in which a given volume of dry air becomes satu- 

 rated with vapour (or saturated within a given relative degree) 

 is nearly independent of the temperature, the source of vapour 

 being constant. 



Experiment IX. — A large bottle, containing fifteen half-pints, 

 was provided with a thermometer, as in Experiment III. The 

 air in the bottle was dried in the following manner : — The bottle 

 A, containing the air to be dried, was provided 

 with a cork fitting the mouth of the bottle C, /^^^T^ 

 containing a considerable portion of strong \ 

 sulphuric acid, as shown in the diagram. The \ Iffi^ 

 apparatus remained in this state for several \_/ 

 hours, the acid being occasionally agitated to 45^ 



secure the absorption of all the watery vapour ^^^fej 3 

 in the air. The thermometer, with a damp 

 calico cap upon its bulb, was then placed in §|jllifi§U^- 

 this dry-air bottle, and the times correspond- Jj [ «ypP 

 iug to different points of depression were duly 

 observed. After the maximum point of depression had been 

 attained, the thermometer gradually rose until the air in the 

 bottle became saturated with vapour, and then the temperature 

 became the same as the surrounding air. It is presumed that 

 the temperature indicated by the thermometer in the bottle, 

 towards the close of the experiment, corresponds with the rate 

 of evaporation going on from the damp calico, and therefore be- 

 comes a measure of the degree of saturation of the air in the 

 bottle at the instant of observation. The following are the 

 results of two experiments at the temperatures of 7° and 13 c C. 



At 7° the maximum depression was found to be 3 C, 2, the 

 corresponding time being 8*5 minutes; the times correspond- 

 ing to o, 9 and o, 6 below the temperature of saturation were 

 found to be 70 and 90 minutes respectively. 



At 13° the maximum depression was found to be 5°, the 



