i88o.] 
On Water and Air. 
123 
that continuousness of the vein is an illusion, and it has 
been the subject of examination by very distinguished men. 
What is the cause of this change in the character of the 
vein from steadiness to unsteadiness ? Mr. Cottrell has 
here arranged an experiment which shows a very remarkable 
property of fluid bodies, and which illustrates the shape 
which they assume if left free to choose their own shape. 
He is going to place some oil in a liquid in this vessel. It is 
very difficult to arrange this experiment. The liquid ought 
to be of exactly the same weight or specific gravity as the 
oil, and if it be so arranged you will find that when the olive 
oil is placed in this liquid it will exactly float in the liquid, 
no matter where you place it, without sinking and without 
rising, and you will find that the olive oil, free to choose its 
own shape, becomes a perfect sphere. The rain which you 
saw descending to-day when you were coming here appeared 
to be descending in lines or streaks. If such a rain shower 
could be illuminated with a single flash of lightning on a 
dark night you would see the rain drops, as perfedt spheres, 
standing still in the air. The reason that they appear as 
streaks in the air is that the impression made upon the 
retina of the eye by every drop of rain lasts for a little time, 
I have to thank my excellent friend Professor Dewar for 
giving you an illustration of what I now want to show you. 
There is an egg. That egg would sink in water. It would 
swim upon salt and water or brine, that is if the brine were 
made very strong. He has here made some brine, and that 
mixture is of exactly the same weight as the egg. The egg 
at present floats at the junction of the two liquids (Fig. 8), 
but it would float in any part of the column of brine where 
it might be placed. [From the bottom of the glass vessel 
to e was filled with the mixture of salt and water ; from e 
upwards was filled with ordinary water ; the egg is shown 
floating at the junction of these two liquids.] 
Here is the olive oil which is arranged in the way that 
has been described. It has been dropped into a mixture of 
alcohol and water, and there you see they assume the 
globular form. If put into alcohol alone the oil would sink 
to the bottom, as it is heavier than alcohol. If poured into 
water the oil would rise to the top. Hence the necessity of 
getting a liquid with a specific gravity something between 
that of alcohol and that of water, in order to cause the 
drops of oil to float in any part of this liquid. 
Now here I have to notice the investigation of that old 
blind philosopher, as Faraday used to call him, Plateau of 
Ghent. He has been blind, I believe, for nearly half a 
