74 VAPORIZATION. 



in a jar of water as in Figure 2, and filling the jar with water in 

 proportion as it rises in the tube.* 



In this instrument we may substitute many other porous 

 substances for the stucco ; but few of them answer so well. 

 Dry and sound cork is very suitable, but permits the diffusion 

 to go on very slowly, not being sufficiently porous ; so do thin 

 slips of many granular foliated minerals, such as flexible mag- 

 nesian limestone. Charcoal, woods, unglazed earthenware, dry 

 bladder, may all be used for the same purpose. 



A slight deviation from the law is observed in gases which 

 differ in a certain physical property from air, namely, in the 

 greater facility with which they may be forced through pores or 

 minute apertures by pressure. A dissimilarity between the 

 gases, in this respect, had long been recognized, although no 

 accurate experiments had been made on the subject. It be- 

 came, however, necessary to examine this point. A small bell- 

 jar, with a short neck and opening at the top, was used, which 

 opening was closed by a plate of stucco half an inch in thick- 

 ness, over which a brass cap and stopcock were fitted and ce- 

 mented. This receiver was placed on the plate of an air-pump 

 in perfect order, and exhausted. When the stopcock was 

 closed, nothing entered the exhausted receiver ; but on opening 

 it, either air entered, forcing its way through the pores of the 

 stucco, or any gas which might be conducted to it, by means of 

 a flexible tube from a proper magazine. Gas was allowed to 

 enter, till it acquired a pressure of three inches, always setting 

 out with air of the tension of one inch mercury in the receiver. 



The same quantity of different gases entered in the following 

 times : 



Air, dry . . . . in 10' 



Air, saturated with moisture, at 60 10' 



Carbonic acid . . . 10' 



Nitrogen . . . . 10' 



Oxygen . . . . 10' 



* A diffusion experiment affords the elements for calculating the specific gravity 

 of a gas. 



r A\2 



The specific gravity = f-\ 



where G is the measure of gas submitted to diffusion, and A the measure of re- 

 turn air. Edinb. Phil, Trans. XII, 222 ; or Phil. Mag. 3rd series, II, 175. 



