Human Physiology. 8$ 



In all elastic bodies there is a partial yielding of the attrac- 

 tive and repelling forces. A steel spring will bend to a certain 

 point, and then it breaks. A thread of india-rubber will 

 stretch several times its usual length, when it becomes rigid 

 like a cord of silk or catgut, and any further strain breaks it. 

 So a gas can be compressed until it becomes a liquid, when it 

 suddenly resists, and further compression requires an immense 

 increase of force. 



Atoms are inconceivably small. Gold plated upon silver 

 wire can be drawn so thin that one foot of wire contains 

 only i-6oooth of a grain of gold; and this can be divided so 

 that i-72,ooo.oooth of a grain is visible to the naked eye, and 

 1-7 2,ooo,ooo,oooth can be seen with the microscope; but this 

 microscopic particle probably consists of millions of atoms. A 

 platinum wire can be drawn to the diameter of i-3oooth of an 

 inch; gold to i-5oooth. And a silver wire 13,000 feet in 

 length, weighing one ounce, is completely gilded by eight 

 : grains of gold. One grain of certain dyes will colour vast 

 quantities of water, through which the myriad atoms must be 

 diffused. Nitrate of silver divided into one hundred billionths 

 is distinctly visible. The stone called tripoli, used for polish- 

 ing metals, is composed of the siliceous sand-like shields 

 of fossil animalculae which are the 1-2 88th of a line in length, 

 and of which there are 41,000,000 to a cubic inch; yet this 

 animal has its organs, and feeds on still smaller animals or 

 vegetables composed of organs, and these of atoms of matter. 



The rocks now filling up the seas on the coasts of the Caro- 

 linas, are composed of shells of se& animalcules, of which there 

 are 3,840,000 in an ounce; yet each of these is composed of an 

 infinity of atoms. The Alps, the Appenines, the stones of the 

 Pyramids, and those of which Paris is built, are composed to a 

 great extent of microscopic shells, secreted by animal exist- 

 ences, which seem to us but a film of slime. A cubic inch of 

 chalk contains the shells of ten millions of highly organised 



