188 SCIENTIFIC AMUSEMENTS. 



ture at which the action takes place, although capable of fusion at 

 a higher temperature. It is a general rule in cases of combination 

 that the product is usually nearer to the solid condition than the 

 materials ; thus when two gases combine together to form a com- 

 pound, that compound is either a solid (Expt. 214), or a liquid 

 which will solidify on further cooling sooner than the original 

 gases (e.g., water produced by the combination of oxygen and 

 hydrogen gases); or a gas which will condense to a liquid and 

 subsequently freeze sooner than the original gases. 



Combination of Gases to form Liquid or Solid Products. 



Expt. 207. To show that Water is produced by burning 

 Hydrogen. Place some granulated zinc (Expt. 16) in the gas 

 generator (fig. 56) ; pour down the thistle funnel some hydrochloric 

 acid solution diluted with two or three times its bulk x)f water ; 

 vigorous effervescence will take place, owing to the dissolving of 

 the zinc by the acid (Expt. 10), and the evolution of hydrogen, in 

 consequence of the chemical action taking place. At first the 

 gas generator contains a quantity of air, so that what will escape 

 by the delivery tube will be a mixture of air and hydrogen ; this 

 mixture has the property of exploding violently when a light is 

 applied to it, so that care must be taken at this stage that the 

 issuing gases are not fired, otherwise the generator will probably 

 be shattered, and damage done by the broken glass and spilt acid. 

 After a few minutes, however, practically all the air originally 

 present will have been expelled, and only hydrogen will issue from 

 the delivery tube ; and at this stage a light may be safely applied 

 to the issuing gas, which will then burn with a scarcely visible 

 blue flame. Over this flame hold a cold dry glass tumbler ; dew 

 will be immediately seen to form on the glass from the condensa- 

 tion of the water vapour formed by the union of the hydrogen with 

 the oxygen of the air, showing that water is a product of the 

 combination. If a large bell jar be used instead of a tumbler, the 

 moisture produced by the burning of the hydrogen will by and by 

 collect to such an extent that distinct drops of water will form and 

 drip down, showing that the dew was really caused by the con- 

 densation of water vapour, and not by any other cause. In order 

 that the gas may burn freely, the delivery tube should be bent so 

 as to point upwards ; the glass should be drawn out in a flame 

 (Expt. 48), and cut off at the tapering part thus produced, so as to 

 leave an orifice considerably smaller than the bore of the original 

 tube before drawing out. To insure that practically all air is 

 expelled from the generator before lighting the issuing gas, a 



