INTRODUCTION. XXXVH 



The Fifteenth Chapter embraces cohesion and capillary 

 attraction, subjects replete with many curious speculations, 

 especially in our investigations of the phenomena of fluids. 

 Whatever may be the cause of fluidity, we know that ice 

 becomes water if a certain degree of he#t be applied to it, and 

 steam if more heat be used. Whether therefore, caloric or motion 

 be the cause of fluidity, we know that in the first instance of 

 the case we have cited, the atoms are fixed in crystals in the 

 second they are thrown into intestine motion and in the third 

 state they are forced asunder with an amazing expansive force. 



Philosophers have usually assumed, that the particles of 

 fluids, since they are so easily moved among one another, are 

 round and smooth. This supposition will account for some 

 circumstances belonging to fluids, as, if the particles are round, 

 there must be vacant spaces between them, in the same manner 

 as there are vacuities between cannon balls when piled toge- 

 ther ; between these balls smaller shot may be placed, and be- 

 tween these, others still smaller, or gravel, or sand, may be 

 diffused. In a similar manner, a certain quantity of particles 

 of sugar can be taken up in a quantity of water without in- 

 creasing the bulk ; and when the water has dissolved the sugar, 

 salt may be dissolved in it, and yet the bulk will not be sensibly 

 augmented ; and admitting that the particles of water are 

 round, this is easily accounted for. Indeed the universal law of 

 gravitation, by which the constituent parts of all bodies mutually 

 attract each other, will cause all such as are fluid, and do not 

 revolve on their own axis, to assume spherical forms. Others 

 have supposed, that the cause of fluidity is the mere want of 

 cohesion of the particles of fluids, which in small quantities, 

 and under peculiar circumstances, arrange themselves in a 

 spherical manner, and form drops. 



Fluids are subject to the same laws with solids. The parts 

 of a solid are so connected as to form a whole, their weight is 

 concentrated in a single point, called the centre of gravity : but 

 the atoms of a fluid gravitate independently of each other, and 

 press not only like solids perpendicularly downwards, but also 

 upwards, sideways, and in every direction. To the flexibility 

 and cohesion of their particles, is owing the singular property 

 which fluids possess of forming themselves into globules, and of 



