q 6 Human Physiology. 



cohere in masses ; so that we have masses of matter, drops of 

 water, lumps of clay, trees, rocks, masses of metal. Two fresh- 

 surfaces of India rubber will adhere to each other; two masses 

 of moist clay or warm wax stick together; dry clay, chalk, etc., 

 require to be pressed together with great force to make a solid 

 body. Two surfaces of perfectly clean, polished plate glass 

 will sometimes cohere. The strength of wood, iron, stone, is 

 in proportion to its molecular cohesive force of the nature of 

 which we have no more idea than we have of gravitation. 



The ultimate atoms of matter have forces as astounding as 

 those of the greatest masses. Atoms of hydrogen and oxygen 

 at a certain temperature unite with a force which produces a 

 burst of flame and a loud noise, which is caused by the pro- 

 duction of a sudden vacuum. But the repulsive forces are still 

 more striking in their results. 



Upon what hypothesis can we explain the force of gunpowder? 

 What so inert as charcoal what safer than sulphur, or saltpetre? 

 Mix them together and touch to them a spark of fire, and we 

 have a thunder that shakes earth and heaven an explosive force 

 that lays a city in ruins. What seems more simple than cotton 

 moistened with acids? We have storehouses filled with both. 

 But, placed in certain conditions, gun cotton has a force ten 

 times as great as gunpowder. And what theory of physics r 

 what calculation of the conservation and correlation of forces 

 gives us the faintest notion of the way in which force is stored 

 up and then let loose in nitro-glycerine, an oily liquid or 

 crystalline solid? Who can explain fulminating silver or 

 mercury? 



Gases, which some consider the natural condition of all 

 matter, solids and liquids being only its transient and acciden- 

 tal states, and all visible and ponderable bodies being but 

 temporary condensations of gases, have some curious properties. 

 They are all soluble in water, seeming to be attracted by its 

 particles, and to fill up their interstices. Water will absorb 



