i 7 6 THE POPULAR SCIENCE MONTHLY. 



in which volume diminishes. The explanation consisted in supposing 

 that a gas is made up of a vast number of minute particles always 

 flying about and striking against one another, and then showing that 

 the rate of impact of such a crowd of particles on the sides of the 

 vessel containing them would vary exactly as the pressure is found to 

 vary. Suppose the vessel to have parallel sides, and that there is only 

 one particle rushing backward and forward between them ; then it is 

 clear that if we bring the sides together to half the distance, the par- 

 ticle will hit each of them twice as often, or the pressure will be 

 doubled. Now, it turns out that this would be just as true for millions 

 of particles as for one, and when they are flying in all directions 

 instead of only in one direction and its opposite; provided only 

 that they interfere with each other's motion. Observe, now : it is a 

 perfectly well-known and familiar thing that a body should strike 

 against an opposing surface and bound off again ; and it is a mere 

 every-day occurrence that what has only half so far to go should be 

 back in half the time; but that pressure should be strictly propor- 

 tional to density is a comparatively strange, unfamiliar phenomenon. 

 The explanation describes the unknown and unfamiliar as being made 

 up of the known and the familiar ; and this, it seems to me, is the true 

 meaning of explanation. 1 



Here is another instance : If small pieces of camphor are dropped 

 into water, they will begin to spin round and swim about in a most 

 marvellous way. Mr. Tomlinson gave, I believe, the explanation of 

 this. We must observe, to begin with, that every liquid has a skin 

 which holds it ; you can see that to be true in the case of a drop, which 

 looks as if it were held in a bag. But the tension of this skin is 

 greater in some liquids than in others ; and it is greater in camphor- 

 and-water than in pure water. When the camphor is dropped into 

 water, it begins to dissolve and get surrounded with camphor-and- 

 water instead of water. If the fragment of camphor were exactly 

 symmetrical, nothing more would happen; the tension would be 

 greater in its immediate neighborhood, but no motion would follow. 

 The camphor, however, is irregular in shape ; it dissolves more on one 

 side than the other ; and consequently gets pulled about, because the 

 tension of the skin is greater where the camphor is most dissolved. 

 Now, it is probable that this is not nearly so satisfactory an explana- 

 tion to you as it was to me when I was first told of it ; and for this 

 reason : By that time I was already perfectly familiar with the no- 

 tion of a skin upon the surface of liquids, and I had been taught by 

 means of it to work out problems in capillarity. The explanation was 

 therefore a description of the unknown phenomenon which I did not 



1 This view differs from those of Mr. J. S. Mill and Mr. Herbert Spencer, in requiring 

 every explanation to contain an addition to our knowledge about the thing explained. 

 Both those writers regard subsumption under a general law as a species of explanation. 

 See also Ferrier's " Kemains," vol. ii., p. 436. 



