ii INTRODUCTION TO MECHANICS. 



We may now proceed to the next general property of bodies, extension. 

 A body which occupies a certain space must necessarily have extension ; 

 that is to say, length, breadth, and depth : these are called the dimensions 

 of extension, and they vary extremely in different bodies. The length, 

 breadth, and depth of a box, or of a thimble, are very different from those 

 of a walking-stick, or of a hair. 



Height and depth are the same dimension, considered in different points 

 of view ; if you measure a body, or a space, from the top to the bottom, 

 it is called depth ; if from the bottom upwards, it is called height. Breadth 

 and width are also the same dimension. 



The limits of extension constitute Jigure or shape : a body cannot be 

 without form, either symmetrical or irregular. Nature has assigned 

 regular forms to her productions in general. The most perfect natural 

 form of mineral substances is that of crystals, of which there is a great 

 variety. Many of them are very beautiful, and no less remarkable by 

 their transparency or colour, than by the perfect regularity of their forms, 

 as may be seen in the various museums and collections of natural history. 

 The vegetable and animal creations appear less symmetrical, and are still 

 more diversified in figure than the mineral kingdom. Manufactured sub- 

 stances assume the various arbitrary forms which the art of man designs 

 for them ; and an infinite number of irregular forms are produced by frac- 

 tures, such as broken china, or glass, or the fragments of mineral bodies, 

 which are broken in being dug out of the earth, or decayed by the effect 

 of torrents and other causes. 



We may now proceed to divisibility ; that is to say, a susceptibility of 

 being divided into an indefinite number of parts. Take any small quan- 

 tity of matter, a grain of sand, for instance, and cut it into two parts ; 

 these two parts might be again divided, had we instruments sufficiently 

 fine for the purpose ; and if, by means of pounding, grinding, and other 

 similar methods, we carry this division to the greatest possible extent, and 

 reduce the body to its finest imaginable particles, yet not one of the 

 particles will be destroyed, and the body will continue to exist, though in 

 this altered state. A single pound of wool may be spun so fine as to 

 extend to nearly 100 miles in length. 



The melting of a solid body in a liquid also affords a very striking 

 example of the extreme divisibility of matter ; when you sweeten a cup of 

 tea, for instance, with what minuteness the sugar must be divided to be 

 diffused throughout the whole of the liquid ! And if a few drops of red 

 wine be poured into a glass of water they will immediately tinge the 

 liquid throughout. The odour of lavender-water, or any other perfume, 

 will be almost as instantaneously diffused throughout the room if the bottle 

 be opened. The odour or smell of a body is part of the body itself, and 

 is produced by very minute particles or exhalations which escape from 

 odoriferous bodies, and come in actual contact with the nose ; and it 

 would be just as impossible to smell a flower, the odoriferous particles 

 of which did not touch the nose, as to taste a fruit, the flavoured particles 

 of which did not come in contact with the tongue. If a bottle of lavender- 

 water be left open a sufficient length of time, the whole of the liquid will 

 evaporate and disappear. But though so minutely subdivided as to be im- 

 perceptible to any of our senses, each particle would continue to exist ; for 

 it is not within the power of man to destroy a single particle of matter; nor 

 is there any reason to suppose, that in nature an atom is ever annihilated. 



When a body is burnt to ashes, part of it, it is true, appears to be 

 destroyed ; the residue of ashes beneath the grate, for instance, is very 



