INTRODUCTION TO MECHANICS; v 



of heat is in no body more sensible than in air, which dilates and con- 

 tracts by its increase or diminution in a very remarkable degree. 



To return to its antagonist, the attraction of cohesion; it is this power 

 which restores to vapour its liquid form, which unites it into drops when 

 it falls to the earth in a shower of rain, and which gathers the dew into 

 brilliant gems on the blades of grass : for rain does not descend from the 

 clouds at first in the form of drops, but in that of mist or vapour, which 

 is composed of very small watery particles; these, in their descent, 

 mutually attract each other, and those that are sufficiently near in con- 

 sequence unite and form a drop, and thus the mist is transformed into a 

 shower. The dew also was originally in a state of vapour, but is, by the 

 mutual attraction of the particles, formed into small globules on the blades 

 of grass : in a similar manner the rain upon the leaf collects into large 

 drops, which, when they become too heavy for the leaf to support, fall to 

 the ground. 



Among the wonderful phenomena of nature, we must not omit to 

 point out a curious effect of the attraction of cohesion. It enables liquids 

 to rise above their level in capillary tubes : these are tubes the bores of 

 which are so extremely small that liquids ascend within them, from the 

 cohesive attraction between the particles of the liquid and the interior 

 surface of the tube. You may perceive the water rising in a small glass 

 tube immersed in a goblet of water. It creeps up the tube to a certain 

 height and there remains stationary, because the cohesive attraction 

 between the water and the internal surface of the tube is balanced by 

 the weight of the water within it. If the bore of the tube were narrower, 

 the water would rise higher ; and if you immerse several tubes of bores 

 of different sizes, you will see it rise to different heights in each of them. 

 In making this experiment, the water should be coloured with a little 

 red wine, in order to render the effect more obvious. 



All porous substances, such as sponge, bread, linen, &c., may be con- 

 sidered as collections of capillary tubes : if you dip one end of a lump of 

 sugar into water, the water will rise in it, and wet it considerably above 

 the surface of that into which you dip it. 



We shall now explain the attraction of gravitation. It is unnecessary 

 here to inquire whether it be only another modification of the same pro- 

 perties which produce the attraction of cohesion, which it certainly 

 resembles in this, that it really results from the attractive force of the 

 minute particles of matter of which bodies are composed. But, tracing it 

 only in its effects, we now speak of it as a force acting, unlike that of 

 cohesion, at considerable distances, and only perceptible in its effects 

 when many particles of matter are combined together in one mass. It 

 acts therefore on the largest bodies, and at immense distances as well as 

 small ones. Let us take, for example, one of the largest bodies in na- 

 ture, and observe whether it does not attract other bodies. What is it 

 that occasions the fall of a book when it is no longer supported? You 

 will say that all bodies have a natural tendency to fall. That is true ; 

 but that tendency is produced by the attraction of the earth. The earth, 

 being much larger than any body on its surface, forces every other, which 

 is not supported, to fall to it. 



When you are accustomed to consider the fall of bodies as depending 

 on this cause, it will appear to you as natural, and surely much more 

 satisfactory, than if the cause of their tendency to fall were totally 

 unknown. Thus all matter is attractive, from the smallest particle to the 

 largest mass ; and bodies attract each other with a force proportioned to 

 the quantity of matter they contain. 



