HYDROGEN. 147 



bringing the surfaces of the liquids in the test-tube and beaker on a level, and 

 marking on the outside of the test-tubes (with a file or paper strip) the exact 

 height of the gas. 



After having emptied the test-tubes, they may be filled with water from a 

 pipette or from a burette to the point which has been marked, and thus the 

 exact volume of gas generated is ascertained. 



Kepeat the operation, using 0.065 gramme of zinc and 0.024 gramme of mag- 

 nesium. Notice that in this case equal volumes of hydrogen are obtained. 

 Calculate the weight of hydrogen from the cubic centimetres liberated, and 

 compare this weight with the weights of zinc and magnesium used. What 

 relation is there between the weights of the liberated hydrogen and the metala 

 used, and the atomic weights of these three elements ? 



Properties. Hydrogen is a colorless, inodorous, tasteless gas ; it 

 is the lightest of all known substances, having a specific gravity of 

 0.0695 as compared with atmospheric air ( = 1). One liter of hydro- 

 gen at C. (32 F.), and a barometric pressure of 760 mm., weighs 

 0.08987 gramme, or one gramme occupies a space of 11.127 liters; 

 100 cubic inches weigh about 2.265 grains. 



Hydrogen and helium resist liquefaction more than other gases. Hydro- 

 gen has been liquefied by causing the gas, cooled to a temperature of 

 205 C. ( 337 F.), to escape under certain conditions from a vessel in 

 which it was stored at a pressure of 180 atmospheres. Liquid hydrogen is 

 clear and colorless ; it has a sp. gr. of 0.07, and boils at 253 C. (- 423 F.), 

 under normal atmospheric pressure ; it also has been solidified lately, and the 

 temperature reached is thought to be about 256 C. (428 F.). 



In its chemical properties, hydrogen resembles the metals more than 

 the non-metals ; it burns easily in atmospheric air, or in pure oxygen, 

 with a non-luminous, colorless, or slightly bluish flame producing 

 during this process of combustion a higher temperature than can be 

 obtained by the combustion of an equal weight of any other substance. 



Two volumes of hydrogen combine with one volume of oxygen, 

 forming two volumes of gaseous water, and the formation of water 

 by the combustion of hydrogen distinguishes it from other gases. 



The chemical affinity which hydrogen possesses for oxygen is so 

 great that it abstracts the oxygen from many oxides. Thus, if 

 hydrogen at a red heat be passed over the oxides of copper or iron 

 the metals are set free, while water is formed : 

 CuO + 2H = H 2 + Cu. 



This process of abstracting oxygen from an oxide is called reduc- 

 tion or deoxidation, and substances having the power of accomplish- 

 ing this result are called reducing or deoxidizing agents. Hydrogen, 

 consequently, is a reducing agent. . 



