of the Galvanic Battery. 455 



that the existence of matter may just as well be denied as the 

 existence of atoms. 



I felt it my duty to be the more explicit upon this point, because 

 I believe that we haye to look to the further development of the 

 atomic theory for the chief progress of scientific chemistry. I 

 have to speak to you not only of atoms as really existing, but 

 also of the movements of those atoms as productive of various 

 important phenomena. It is now a good many years since I 

 showed that chemical decompositions are the results of preexist- 

 ing atomic movements, in fact that the force which produces 

 chemical change is no other than a particular form of atomic 

 motion. Some of you may probably feel a little hesitation in 

 admitting that, while a given mass of matter remains in the same 

 place, the atoms of which it is composed are yet in a state of in- 

 tensely rapid vibration. Let us take the example of a gas, say 

 common air. If air really consists of little atoms in a state of 

 rapid vibration striking against the sides of any vessel in which 

 the air is confined, and rebounding in virtue of their elasticity, 

 what must we expect to happen if air was introduced into a ves- 

 sel having a porous side, or a side with a number of very small 

 holes in it ? Of course the atoms would pass through the small 

 holes and get out. 



In illustration of this, I will show you one of those beautiful 

 phenomena of so-called Diffusion which have attracted the atten- 

 tion of so many able investigators of late years, especially of my 

 distinguished friend the Master of the Mint. I have here a 

 vessel of which the upper part consists of a porous material, 

 viz. unglazed earthenware. The vessel contains air, and I assert 

 that the atoms of which this enclosed air consists are striking 

 against the porous sides of the vessel and escaping through 

 them. But the outer air is getting into the vessel as fast as the 

 inner air gets out, so that the vessel does not get emptier or 

 fuller, and the blue liquid in this tube, which is really the neck 

 of the bottle, remains unmoved. I will now put the apparatus 

 (still full of air) into a chamber full of a gas consisting of much 

 lighter and more moveable atoms than those of air, in fact into 

 a chamber full of hydrogen, and you see already that the blue 

 liquid is being driven out by the neck of my vessel. This 

 proves to us that the light atoms of hydrogen are getting in 

 through the porous sides quicker than the heavy atoms of air 

 get out. Each atom of hydrogen weighs only y^th as much 

 as an atom of nitrogen, which is the chief constituent of air : 

 but it gets no benefit from its lightness beyond the power 

 of moving faster, and I will prove to you that gases need not 

 differ in density in order to diffuse into one another, by filling 

 this bell jar with a gas called nitric oxide, which is very nearly 



