DIVISIBILITY. 33 



gramme of the colouring matter, the whole liquid in the 

 tube will contain less than the T^,Wo,TTo tn P art f a 

 gramme. A square centimetre of the finest gold-leaf 

 weighs about ^ /oVo^h f a gramme, and a small piece of 

 it, much less than a square millimetre, hence much less 

 in weight than the 5-oovoo"o tn P art f a gramme of 

 gold, is easily visible to the naked eye. Portions of 

 bodies, much smaller still than those described, may be 

 rendered visible by proper means; but whatever means 

 may be employed, they are insufficient to divide bodies 

 into the smallest particles 'of which they consist. 



These smallest indivisible particles are called mole- 

 cules, and the forces of cohesion and expansion, which 

 induce these particles either to cohere or to recede 

 from each other (art. 4), are consequently termed mole- 

 cular forces. 



A narrow piece of glass tubing for this experiment may be 

 obtained by drawing out a wider tube before the lamp. The 

 middle portion of a piece of tubing, 12 or 15 cm long, and from 5 to 

 7mm w id e ^ i s heated in the flame of the spirit lamp until it is quite 

 soft ; it is then quickly removed and pulled with both hands in 

 the direction of its length until the middle portion has the required 

 width. After cooling, this portion is removed by making a scratch 

 with the three-cornered file at each extremity, and breaking the 

 intermediate portion away. The tube should be constantly turned 

 on its axis while it is in the flame, otherwise it will not be heated 

 with sufficient uniformity, and cannot be drawn out properly. 



7. Gravity. Absolute and Specific Weight. A stone 

 cannot be lifted from the ground, or a book from the 

 table, without using a certain amount of force, which is 

 less or greater according as the mass of the body is less' 

 or greater. If left to itself the raised body will speedily 

 fall back again towards the ground until prevented 

 from falling further by some obstacle, that is, until it 



D 



