LECTURES. 195 



Ehrenberg has found whole rocks composed of the shells of animals 

 so minute that one cubic line contains about 23 millions of them. 

 These animals must have had limbs and other parts. 



(18.) Divisibility of odorous matter. 



Our olfactory nerves frequently detect the presence of matter in the 

 atmosphere, of which no chemical test could afford an indication, 

 A single grain of musk will scent a large room for years. 

 The dog hunts by the scent of odors imperceptible to man. 

 A single drop of lavender made to fill a large room. 



POROSITY AND COMPRESSIBILI'lY. 



(19.) All bodies can be indefinitely compressed, or reduced in 

 volume ; consequently, in their ordinary state, they do not form a 

 plenum of mutter . 



The intervals between the parts are called pores. 



If we adopt the hypothesis of the atomic constitution of matter, we 

 must admit the existence of different orders of pores. 



Pores between the atoms — between the molecules — between the- 

 particles, and between the grosser parts. 



Illustrations— shrinking by cold — mixture of liquids — water in,. 



Sponge. ■ . I n ' 



Improper idea often given of porosity.. ^ 

 (20.) Jieal and apparent volume of bodiGs. 

 Method of determining the ratio of these. 

 The sum of all the atoms of a body constitutes its mass^. 

 Density is the quantity of matter in a given bulk or volume.. 

 In homogeneous bodies mass proportion tothe bulk. 

 In heterogeneous bodies, to bulk and density. 

 (21.) Absolute quantity of matter in a given body may be exceed- - 



ingly small. 



Illustration— vessel filled with alcohol, great quantity ot cotton 

 introduced— sponge dipped in vessel nearly filled with water. 



Relative bulk of steam and water ; great porosity of the former. 



(22.) Porosity of organized bodies. 



Mercury forced through a cylinder of oak— pine sinks in water when 

 saturated by pressure — experiments of Scoresby. Skin perforated with, 

 a thousand holes in the length of an inch, through which the insen^ 

 sibb perspiration passes ; water through a bladder. Remarks on 

 India rubber cloth — improper for clothing. 



(23.) Porosity of minerals. 



Mercury through lead ; condensation of alloys ; water through 

 gold ; water through cast iron ; gold leaf translucent. Porosity of 

 chalk — of marble — of hydrophane. 



Method of coloring agate. 



Eflects of water on rocks. Formation of stalagmites and stalactites. 



Method of determining whether a stone will stand the eJOfects of 

 frost by the absorption and crystallization of a salt. 



