270 THE POPULAR SCIENCE MONTHLY. 



directions. All these molecules are constantly changing the direc- 

 tion of each other's motion ; they are flying about with very different 

 velocities, although, as I have said, their mean velocity is about sev- 

 enteen miles a minute. If the velocities were all marked off on a 

 scale, they would be found distributed about the mean velocity just 

 as shots are distributed about a mark. If a great many shots are 

 fired at a target, the hits will be found thickest at the bull's-eye, and 

 they will gradually diminish as we go away from that, according to 

 a certain law, which is called the law of error. It was first stated 

 clearly by Laplace ; and it is one of the most remarkable consequeiices 

 of this theory that the molecules of a gas have their velocities dis- 

 tributed among them precisely according to this law of error. In 

 the case of a liquid, it is believed that the state of things is quite 

 different. We said that in the gas these molecules are moved in 

 straight lines, and that it is only during a small portion of their 

 motion that they are deflected by other molecules ; but in a liquid we 

 may say that the molecules go about as if they were dancing the 

 grand chain in the Lancers. Every molecule after parting company 

 with one finds another, and so is constantly going about in a curved 

 path, and never gets quite clear away from the sphere of action of 

 the surrounding molecules. But, notwithstanding that, all molecules 

 in a liquid are constantly changing their places, and it is for that 

 reason that diffusion takes place in the liquid. Take a large tank of 

 water and drop a little iodine into it, and you will find after a certain 

 time all the water turned slightly blue. That is because all the iodine- 

 molecules have changed like the others and spread themselves over 

 the whole of the tank. Because, however, you cannot see that, except 

 where you use different colors, you must not suppose that it does not 

 take place where the colors are the same. In every liquid all the 

 molecules are running about and continually changing and mixing 

 themselves up in fresh forms. In the case of a solid quite a different 

 thing takes place. In a solid every molecule has a place which it 

 keeps ; that is to say, it is not at rest any more than a molecule of a 

 liquid or a gas, but it has a certain mean position which it is always 

 vibrating about and keeping fairly near to, and it is kept from losing 

 that position by the action of the surrounding molecules. These are 

 the main points of the theory of the constitution of matter as at pres- 

 ent believed. It differs from the theory of Democritus in this way. 

 There is no doubt that in the first origin of it, when it was suggested 

 as a whole, it was a guess of his. 



In order to make out that your supposition is true, it is necessary 

 to show, not merely that that particular supposition will explain the 

 facts, but also that no other one will. Now, by the efforts of Clarges 

 and Prof. Clerk Maxwell, the molecular theory of matter has been 

 put in that other position, namely, instead now of saying, "Let 

 us suppose that such and such tilings are true," and then deducing 



