6 THE DATA OF BIOLOGY. 



carbonic acid, containing an additional atom of oxygen (C O2) 

 assumes a liquid form only under a pressure of nearly forty 

 atmospheres. The several compounds of oxygen with 



nitrogen, present us with an instructive gradation. Protoxide 

 of nitrogen, which contains one atom of each element (N O), 

 is a gas condensible only under a pressure of some fifty at- 

 mospheres ; deutoxide of nitrogen (N O2) is a gas hitherto 

 uncondensed (the molecular mobility remaining undiminished 

 in consequence of the volume of the united gases remaining 

 unchanged) ; nitrous acid {N O3) is gaseous at ordinary 

 temperatures, but condenses into a very volatile liquid at the 

 zero. of Fahrenheit; peroxide of nitrogen (N O4) is gaseous at 

 71°, liquid between that and 16°, and becomes solid at a tem- 

 perature below this ; while nitric acid (N O5) may be obtained 

 in crystals which melt at 85° and boil at 113°. In this 

 series we see, though not with complete uniformity, a de- 

 crease of molecular mobility as the weights of the compound 

 molecules are increased. The hydro-carbons illus- 



trate the same general truth still better. One series of them 

 will suffice. Marsh gas (C2 H4) is permanently gaseous, 

 defiant gas (C4 H4) may be liquefied by pressure. Oil 

 gas, which is identical with defiant gas in the proportions 

 of its constituents but has double the atomic weight, (Cg Hg), 

 becomes liquid without pressure at the zero of Fahrenheit. 

 Amylene (Cio Hio) is a liquid which boils to 102°. And the suc- 

 cessively higher multiples, caproylene (C12 Hio), caprylene 

 (C16 Hie), elaene (Cis His) and paramylene (C20 Hgo), are liquids 

 which boil respectively at 102°, 131°, 25Y°, 230°, and 329°. 

 Cetylene (O32 H32) is a liquid which boils at 527° ; while pa- 

 raffine (C54 H54) and mylene (Ceo Hgo) are solids. Only 



one compound of hydrogen with nitrogen has been obtained 

 in a free state — ammonia (H3 N) ; and this, which is gaseous, 

 is liquefiable by pressure, or by reducing its temperature to 

 — 40° F. In cyanogen, which is composed of nitro- 



gen and carbon (N C2), we have a gas that becomes liquid at a 

 pressure of four atmospheres and solid at — 30° F. And, in 



