COMBUSTION. 149 



an apparatus is called a " hydrogen pistol," but it ought 

 really to be called an " oxyhydrogen pistol," for only a 

 mixture of these gases explodes. The two gases can be 

 mingled in a bag, and by the aid of a common tobacco-pipe, 

 as seen in Fig. 64, soap-bubbles can be 

 formed, which on flying upward can be ex- 

 ploded by touching them with a light. To 

 obtain good soap-bubbles mix a little glyc- 

 erine with the soap-water before using. In 

 such experiments we use in bulk twice as 

 much hydrogen as oxygen, for it is in this proportion that 

 these gases unite to form water ( 140). Common air is 

 often used in place of oxygen, and answers the purpose be- 

 cause it contains this gas. When this is used we introduce 

 about equal bulks of the air and the gas. In all such ex- 

 periments great care should be exercised. For example, in 

 the bubble experiment we should be careful not to bring 

 the light near the pipe of the gas-bag, else the whole might- 

 be exploded at once. 



192. Spontaneous Combustion. We mean by spontane- 

 ous combustion the taking fire of any substances without 

 the application of heat to them. We will give you some ex- 

 amples, and explain them. If you place a bit of phosphor- 

 us of the size of a pea upon blotting-paper, and sprinkle 

 over it some soot or powdered charcoal, it after a while 

 melts and bursts into a flame. This is owing to the large 

 absorption of oxygen gas by the carbon, as noticed in 96. 

 Much oxygen is thus introduced to the phosphorus, for 

 which it has a strong affinity ; and a union is therefore 

 readily effected between them, which union is combustion. 

 Heat is generated by the absorption of the gas ; and as car- 

 bon is a non-conductor, the heat is retained, and is sufficient 

 to start the combustion of the phosphorus. Indeed, where 

 there is a large amount of powdered charcoal heaped to- 



