148 CHEMISTRY. 



luminous by merely striking against a solid substance, with- 

 out in the least altering it, confirms what we have before 

 learned, that the light-giving power of flame is dependent 

 chiefly on the presence of incandescent solids. 



Instead of using pure oxygen and pure hydrogen, an ex- 

 cellent light for all practical purposes is obtained by em- 

 ploying oxygen and coal gas. Strong metallic cylinders 

 (see Fig. 62, p. 147) containing these gases under pressure 

 are now sold in the large cities to any one wanting a bright 

 light or intense heat. This light, also called the oxycalcium 

 light, is very frequently seen in theatres, torchlight proces- 

 sions, and even as a' means of advertising. The stereopti- 

 con used in illustrating public lectures is simply a magic- 

 lantern provided with a calcium light. 



190. Cause of Explosions. So long as the two gases hy- 

 drogen and oxygen are kept separate before burning them 

 no explosion takes place ; but if oxygen and hydrogen be 

 mingled together and then fire be applied there is a violent 

 action, and a report proportioned to the amount of the gases. 

 The combustion is alike in both cases, oxygen and hydro- 

 gen uniting to form water, and the explosion is due to the 

 sudden expansion of the gases caused by the intense heat 

 generated by their chemical union. The noise is pro- 

 duced by the sudden collision of the instantaneously ex- 

 panded vapor with the air surrounding the vessel contain- 

 ing it. 



191. Experiments. Some interesting experiments can be 

 tried illustrative of the explosive combustion of gases. If 



into a strong brass vessel, a, Fig. 63, we in- 

 troduce a mixture of oxygen and hydrogen, 

 and, having pushed the cork, c, in tightly, 

 pass electricity by the ball and wire at b, an 

 explosion will occur. The cork will be violently driven out 

 by the expansive force of the heated vapor produced. Such 



