202 LECTURES. 



Table of specific gravities exhibited. Hydrogen the lightest sub- 

 stance, and Iridium the heaviest — the first is .069 the weight of air, 

 and the latter 23 times that of water. In equal bulks the weight of 

 the latter is more than a quarter of a million of times that of the 

 former. [Dr. Hare.] 



(55.) It is a remarkable fact that the inertice of equal bulks of dif- 

 ferent substances are in the same ratio as their weights. Hence the 

 masses, the quantities of matter, and the densities of different sub- 

 stances of the same bulk are said to be proportional to their relative 

 weights ; or, in other words, to their specific gravities. 



(56.) The absolute weight in ounces of solids and liquids may be 

 obtained by the following relation, in which B is the bulk in cubic feet, 

 and S the specific gravity. 



W=1000SxB. (5.) 



The weight of air shown. 770 times lighter than water at the 

 freezing point with bar. at 30 inches. Difference between the weight 

 of air and the pressure of the atmosphere. 



Electrical and magnetic attraction and repulsion. 



(57.) These are exhibited under certain conditions in all kinds of 

 matter. They will be fully discussed under their appropriate heads. 

 Magnetic attraction may however be here employed to illustrate the 

 general principles of polarity. 



The attraction and repulsion of a magnet shown to exist at its two 

 ends called poles; hence the term polarity — origin of the name — 

 neutral point at the middle between the two poles — the magnet 

 broken, each part shown to be a perfect magnet with attracting and 

 repelling poles — again each part divided into two pieces, and again 

 the exhibition of new poles, and so on, until we infer that the polarity 

 exists in every part of the mass, or in other words that attraction and 

 repulsion belong to the opposite extremes of every molecule of the 

 metal. 



(58.) In order to explain the phenomena of chemical saturation, 

 crystalization, the difference of the liquid and solid states of bodies, 

 as well as other phenomena we are obliged to admit a kind of polarity 

 as a general property of the molecules of all bodies. 



Attraction of cohesion and adhesion, or the molecular forces. 



(59.) By cohesion we designate the force by which the parts of the 

 same body are held together^ and by adhesion that which causes the 

 parts of dissimilar bodies to unite. These forces are also sometimes 

 distinguished by the names of homogeneous and heterogeneous at- 

 traction. 



(60.) There is also between the molecules of the same body and the 

 parts of different bodies a repulsive action and this with the attrac- 

 tions constitute what are called the molecular forces. 



Also sometimes called corpuscular action. 



