442 Mr. Anderson [April 9, 



a second prism be interposed and placed at right angles to the first, 

 the light will be unable to get through ; but if we introduce between 

 the crossed Nicols a substance capable of turning the plane of vibra- 

 tion again, then a certain portion of the light will pass. I have now 

 projected on the screen the feeble light emerging from the crossed 

 Nicols. I introduce the microscopic preparation of cork cells between 

 them, and you see the crystals glowing with many-coloured lights on 

 a dark ground. 



Minute though these crystals are, they are very numerous and 

 hard, and it is partly to them that is due the extraordinary rapidity 

 with which cork blunts the cutting instruments used in shaping it. 

 Cork-cutters always have beside them a sharpening stone, on which 

 they are obliged to restore the edges of their knives after a very few 

 cuts. 



The cells of the cork are filled with gaseous matter, which is 

 very easily extracted, and which has been analysed for me by Mr. 

 G. H. Ogston, and proved to be common air. I have here 

 a glass tube in which are some pieces of cork which have been 

 cut into slices so as to facilitate the escape of the air. I 

 connect the tube with an exhausted receiver and project the image on 

 the screen ; you see rising from the cork bubbles of air as numerous, 

 but much more minute than the bubbles which rise from sparkling 

 wines ; much more minute, because the bubbles you see are expanded 

 to seven or eight times their volume at atmospheric pressure, on 

 account of the vacuum existing in the tube. The air will continue 

 to come off for an hour or more, and from measurements made by 

 Mr. Ogston I find that the air occluded in the cork amounts to 

 about 53 per cent, of its volume. The facility with which the air 

 escaj^es, compared with the impermeability of cork to liquids, is very 

 remarkable. 



I throw on the screen the image of a section cut from a cork which 

 was kept under a vacuum of about 26 inches for five days and nights ; 

 aniline dye was then injected, and yet you see that the colour has 

 not more than permeated the outermost fringe of cells, those, in fact, 

 which had been broken open by the operation of cutting the cork. 

 By keeping cork for a very long time in an almost perfect vacuum, 

 and then injecting dye, a slight darkening of the general colour of a 

 section of the cork may be noticed, but it is very slight indeed. How, 

 then, does the air escape so readily when the cork is placed in vacuo ? 



The answer is, that gases possess the property of diffusion, that is, 

 of passing through porous media of inconceivable fineness. When 

 two gases, such as hydrogen and air, are separated by a porous medium, 

 they immediately begin to pass into each other, and the lighter gas 

 passes through more quickly than the heavier. 



I have here a glass tube, the upper end of which is closed by a 

 thin slice of cork, the lower end dips into a basin of water. Some 

 hours ago the tube Avas filled with hydrogen, which you know is about 

 fourteen and a half times lighter than air, conse(j[uently, according to 



