THE FIRST AND THE LAST CATASTROPHE. 277 



ment rests that the molecules of a given gas are all very nearly of the 

 same weight. Why do I say very nearly? Because evidence of that 

 sort can never prove that they are exactly of the same weight. Tlie 

 means of measurement we have got may be exceedingly correct, but a 

 certain limit must always be allowed for deviation ; yet if the deviation 

 of molecules of oxygen from a certain standard of weight were very 

 small, and restricted witliin small limits, it would be quite possible 

 for our experiments to give us the results which they do now. Sup- 

 pose, for example, the variation in the size of the oxygen-atoms was 

 as great as that in the weight of different men, then it would be very 

 difficult indeed to tell by such a process of sifting what that differ- 

 ence was, or in fact to establish that it existed at all. But, on the 

 other hand, if we suppose the forces which originally caused all those 

 molecules to be so nearly alike as they are, to be constantly acting 

 and setting the thing right as soon as by any sort of experiment we 

 set it wrong, then the small oxygen-atoms on one side would be made 

 up to their right size, and it would be impossible to test the difference 

 by any experiment which was not quicker than the processes by which 

 they were made right again. 



There is another reason why we are obliged to regard that experi- 

 ment as only aj)proximate, and as not giving us any exact results. 

 There is very strong evidence, although it is not conclusive, that in a 

 given gas say in a vessel full of carbonic acid the molecules are not 

 all of the same weight. If we compress the gas, we find that when in 

 the state of a perfect gas, or nearly so, the pressure increases just in 

 the ratio that the volume diminishes. That law is entirely explained 

 by means of the molecular theory. It is what ought to exist if the 

 molecular theory is true. If we compress the gas further, we find 

 that the pressure is smaller than it ought to be. This can be ex- 

 plained in two ways : First of all we may suppose that the molecules 

 are so crowded that the time during wliich they are sufiiciently near 

 to attract each otlier sensibly becomes too large a ))roportion of the 

 whole time to be neglected ; and this will account for the change in 

 the law. There is, however, another explanation. We may suppose, 

 for illustration, that two molecules approach one another, and that 

 the speed at which one is going relatively to the other is very small, 

 and then that they so direct one another that they get caught together, 

 and go on circling, making only one molecule. This, on scientific 

 principles, will account for our fact, that the pressure in a gas which 

 is near a liquid state is too small ; that instead of the molecules going 

 about singly, some are hung together in couples and some in larger 

 numbers, and making still larger molecules. This supposition is con- 

 firmed very strikingly by the spectroscope. If we take the case of 

 chlorine gas, we find that it changes color that it gets darker as it 

 approaches the liquid condition. This change of color means that 

 there is a change in the rate of vibration which belongs to its compo- 



