6 THE DATA OF BIOLOGY. 



by the union of oxygen with carbon, the first, called carbonic 

 oxide, which contains one atom * of carbon to one of oxygen 

 (expressed by the symbol CO) is a gas condensible only with 

 great difficulty; and the second, carbonic acid, containing an 

 additional atom of oxygen (C 2 ) assumes a liquid form also 

 only under a pressure of about forty atmospheres. The 



several compounds of oxygen with nitrogen, present us with 

 an instructive gradation. Nitrous oxide (N" 2 0), is a gas 

 condensible only under a pressure of some fifty atmospheres; 

 nitric oxide (NO) is a gas which although it has been lique- 

 fied does not condense under a pressure of 270 atmospheres at 

 46.4 F. (8 C.) : the molecular mobility remaining un- 

 diminished in consequence of the volume of the united gases 

 remaining unchanged. Nitrogen trioxide (N 2 3 ) is gaseous 

 at ordinary temperatures, but condenses into a very volatile 

 liquid at the zero of Fahrenheit; nitrogen tetroxide (N 2 4 ) 

 is liquid at ordinary temperatures and becomes solid at the 

 zero of Fahrenheit; while nitrogen pentoxide (N 2 5 ) may 

 be obtained in crystals ^rhich melt at 85 and boil at 113. 

 In this series we see, though not with complete uniformity, 

 a decrease of molecular mobility as the weights of the com- 

 pound molecules are increased. The hydro-carbons 

 illustrate the same general truth still better. One series of 

 them will suffice. Marsh gas (C H 4 ) is gaseous except under 

 great pressure and at very low temperatures. Olefiant gas 

 (C 2 H 4 ) and ethane (C, H e ) may be readily liquefied by pres- 

 sure. Propane (C 3 H 8 ) becomes liquid without pressure at 

 the zero of Fahrenheit. Hexane (C 5 H 12 ) is a liquid which 

 boils at 160. And the successively higher multiples, heptane 

 ( C 7H 16 ), octane (C 8 H 18 ), and nonane (C 9 H 20 ) are liquids 

 which boil respectively at 210, 257, and 302. Pentadecan 

 ( C i5 H 32 ) is a liquid which boils at 270, while paraffin-wax, 



"Here and hereafter the word "atom" signifies a unit of something 

 classed as an element, because thus far undecomposed by us. The word must 

 not be supposed to mean that which its derivation implies. In all probability 

 it is not a simple unit but a compound one. 



