PHYSICAL PROPERTIES. 
13 
reflects different colours from different parts of its 
surface. Immediately before the bubble bursts, a 
black spot may be observed near the top. At this 
part the thickness has been proved not to exceed 
the 2,500,000th of an inch. Many insects have trans- 
parent wings so thin, that 50,000 of them placed 
over each other would not form a piece a quarter 
of an inch in height. 
In the organized world we have still more stri- 
king examples of the extreme minuteness of matter. 
The blood which circulates in the arteries and veins 
of animals is not a uniformly red fluid, but consists 
of a watery portion called serum, containing small 
red globules. These globules are found to differ in 
size and shape in different species. Thus, in man, 
and in all animals that suckle their young, they are 
round or spherical ; in birds and fishes, of an oblong, 
spheroidal form. In man, these globules are about 
the 4000th part of an inch in diameter ; and it can 
be shown, that in the small drop of blood which 
remains suspended from the point of a fine needle, 
there must be contained more than a million of these 
globules. Again, animalcules have been discover- 
ed whose magnitude is such that a miUion of them 
does not exceed the bulk of a grain of sand ; and 
yet, by means of the solar microscope, we find that 
these creatures, such as are even invisible to the 
naked eye, are composed of bones, and muscles, 
and bloodvessels ; that they have life and motion, 
sense and instinct, and, in short, are as curiously 
organized as animals of a larger size. 
We have stated that the volume of a body is the 
quantity of space included within its external sur- 
faces, and that its mass is the collection of atoms or 
particles of which it consists. Now if these atoms 
are actually in contact, the volume, of course, will 
be completely occupied by the mass, so that the 
greater the density the less will be the porosity. 
But there is reason to believe that this is not the 
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