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 million 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 staled 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 



