584 THE POPULAR SCIENCE MONTHLY. 



ance very much increased ; in front of the spoon a little heap of 

 liquid gathers, which subsides hut slowly, and there is a depression 

 behind which is as slowly filled up. It is evident that there is some 

 difference between the interior constitutions of the treacle and the 

 water; and that difference consists in this, that the particles of which 

 the treacle is composed move among themselves with very much less 

 facility than do those of the water. The fact, then, of one part of a 

 liquid moving more or less easily among the other parts is that which 

 distinguishes one from another in respect to their viscosity. In a 

 very viscous body, like treacle, the parts move with difficulty ; and in 

 a non-viscous liquid, like water, they move with comparative ease. 

 The fact which I wish to impress upon you this evening is, not that 

 one kind of liquid differs from another; but that one part of a liquid 

 may differ from another in respect of viscosity ; and that as a general 

 rule the surface is more or less viscous than the interior. I will now 

 show you an experiment which will illustrate this fact in a very strik- 

 ing way. I have in a glass vessel a little magnet, which, when I bring 

 near to it a large magnet, will easily and readily follow its motions. 

 The vessel also contains a mixture of water and a substance called 

 saponine. This saponine is extracted from the horse-chestnut, and is, 

 as far as I know, chiefly interesting on account of the extraordinary 

 effect it produces on water when mixed with it. In making the mixt- 

 ure, I have added only one part of saponine to sixty of water, and, 

 to look at, it retains the properties of water ; it is colorless ; it has 

 none of the viscosity of treacle. In. fact, the saponine has next to no 

 effect on the interior parts of the water, but it has a most extraordi- 

 nary and mai-ked effect on the surface ; and that I will now try to 

 illustrate. You now see upon the screen the image of the magnet, 

 and the vessel at the bottom of which there is the mixture of saponine 

 and water. The magnet is at present about an eighth of an inch 

 above the liquid. I bring near the large magnet, and you see how 

 easily it follows its motions. I will now pour in some of my mixture 

 until the magnet lies upon the surface, and I then again bring the 

 large magnet near it. It is iioav upon the surface of the mixture, and 

 you can see some of the bubbles formed as I pour the liquid in. I 

 bring the large magnet as near as it was at first and am moving it, 

 but it produces no effect. I bring it nearer and nearer still no effect. 

 I bring it so near that you can see its shadow, and still the magnet 

 remains absolutely motionless. On the surface of the liquid, then, we 

 have found that the little magnet is totally insensible to the attractive 

 force of this large one. You may say that the same would happen in 

 the case of glycerine or treacle. It might ; but now comes the ex- 

 traordinary part of the experiment. I pour in some more saponine 

 and water, until the little magnet lies a quarter of an inch below its 

 surface; I then bring the large magnet near, and you see the result. 

 Pt moves almost as freely as in the air itself. Hence we have a 



