THE COMPOSITION OF AVATKR. II VI'IK >< , KN 143 



burns with a pale that is, non-luminous flame. 39 Hydrogen does not 

 combine with the oxygon of the atmosphere at the ordinary tempe- 

 rature ; but this combination takes place at a red heat, 40 and is accom- 

 panied by the evolution of much heat. The product of this combination 

 is \vater that is, a compound of oxygen and hydrogen. This is the 

 xy/^/^.v/'x i>f water, and we have already noticed its analysis or decom- 

 position into its component parts. The synthesis of water may be very 

 easily observed if a cold glass bell jar be placed over a burning hydrogen 

 Ha me, and, better still, if the hydrogen flame be lighted in the tube of 

 a condenser. The water will condense in drops as it is formed on the 

 walls of the condenser and trickle down. 41 



Light does not aid the combination of hydrogen and oxygen, so 

 that a mixture of these two gases does not change when exposed to the 

 action of light ; but an electric spark acts just like a flame, and this is 

 taken advantage of for inflaming a mixture of oxygen and hydrogen, or 

 detonating gas, inside a vessel, as will be explained in the following 

 chapters. As hydrogen (and oxygen also) is condensed by spongy 

 platinum, by which a rise of temperature ensues, and as platinum acts 

 by contact (p. 38), therefore hydrogen also combines with oxygen, 

 under the influence of platinum, as Dobereiner showed. If spongy 

 platinum be thrown into a mixture of hydrogen and oxygen, an explo- 

 sion takes place. If a mixture of the gases be passed over spongy 

 platinum, combination also ensues, and the platinum becomes red-hot. 42 



50 If it be desired to obtain a perfectly colourless hydrogen flame, it must issue from 

 a platinum nozzle, as the glass end of a gas-conducting tube imparts a yellow tint to the 

 Hume, owing to the presence of sodium in the glass. 



40 Let us imagine that a stream of hydrogen passes along a tube, and let us mentally 

 divide this stream into several parts, consecutively passing out from the orifice of the 

 tube. The first part is lighted that is, brought to a state of incandescence, in which 

 state it combines with the oxygen of the atmosphere. A considerable amount of heat is 

 e\ -nlved in the combination. The heat evolved then, so to say, ignites the second part of 

 hydrogen coming from the tube, and, therefore, when once ignited, the hydrogen con- 

 tinues to burn, if there be a continual supply of it, and if the atmosphere in which it 

 l)n rns be unlimited and contains oxygen. 



41 The combustibility of hydrogen may be shown by the direct decomposition of water 

 by sodium. If a pellet of sodium be thrown into a cup containing water, then it floats 

 on the water and evolves hydrogen, which may be lighted. The presence of sodium imparts 

 ;i yellow tint to the flame. If potassium be taken, the hydrogen bursts into flame of 

 itself, because sufficient heat is evolved in the reaction for the ignition and inflammation 

 of the hydrogen. The flame is rendered violet by the potassium. If sodium be thrown 

 not on water, but on an acid, it will evolve more heat, and the hydrogen will then also 

 burst into flame. These experiments must be carried on with caution, as sometimes 

 towards the end a mass of sodium oxide (Note 8) is produced, and flies about; therefore 

 it is best to cover the vessel in which the experiment is carried on. 



'- This property of spongy platinum is made use of in the so-called hydrogen cigar- 

 light. It consists of a glass cylinder or beaker, inside which there is a small lead stand 

 i which is not acted on by sulphuric acid), on which a piece of zinc is laid. This zinc is 

 covered by a bell, which is open at the bottom and furnished with a cock at the top. 



