324 



COMBUSTION. 



The light produced by lamps or candles formed of different substances has 

 different illuminating powers, according to the quantities of light evolved by the 

 combination of the gas or vapor with oxygen. 



The vapor of some substances is capable of combining with oxygen at a tem- 

 perature below that which is necessary for the production of flame. Sir Hum- 

 phry Davy coiled a piece of platinum wire round the wick of a spirit-lamp, 

 and, having lighted the lamp, and allowed it to burn till the wire became red 

 hot, he then extinguished it ; the wire, however, with the heat which it had 

 acquired, communicated a sufficient heat to the vapor raised from the alcohol 

 to enable it to combine with the oxygen of the surrounding air ; and a slow 

 combustion, without flame, was thus produced. This process of combustion 

 might be continued for any length of time, or as long as the alcohol in the lamp 

 could supply vapor. 



The product obtained by the combination of oxygen and the vapor of alco- 

 hol in this case was of a nature altogether different from that obtained by the 

 ordinary combustion of the spirit-lamp. Acetic acid forms a part, but not the 

 whole of the product. 



There are other vapors which, like that of alcohol, are susceptible of com- 

 bustion without flame. Among these are the vapors of ether, camphor, and 

 some of the volatile oils. 



If platinum wire, heated to redness, be introduced into a receiver containing 

 a mixture of coal gas, or the vapor of ether, and atmospheric air, it will con- 

 tinue red hot until the whole of the gas is consumed. In this case the gas 

 combines with the oxygen of the atmospheric air with which it is mixed, and 

 combustion takes place. 



Dr. Thomson accounts for this process by the fact of the small specific heat 

 and bad-conducting power of platinum : a small quantity of heat is sufficient to 

 make it red hot, and, being a bad conductor, it loses little heat during the pro- 

 s. Platinum, at a red heat, has a sufficiently high temperature to produce 

 a rapid combination of the vapor of alcohol with oxygen, but it is not sufficient 

 for the production of flame.* 



If a jet of hydrogen gas be projected on a small mass of spongy platinum, 

 the platinum will become red hot, and will continue so as long as the jet plays 

 on it. This forms an easy means of producing an instantaneous light, and an 

 apparatus is constructed in a convenient form for this purpose. By turning a 

 stopcock, the jet of gas is thrown on a small cup containing platinum, which, 

 immediately becoming red hot, is capable of lighting a match. The same 

 effect may be produced by a jet of the gas projected on other substances, such 

 as palladium, rhodium, and iridium. Some others, also, such as osmium, would 

 be attended with a like effect, if their temperatures were previously raised. 

 Platinum foil would not, under these circumstances, redden ; but, if it be 

 crumpled, like paper, it will undergo the same effect as the spongy pla- 

 tinum. 



These effects have been accounted for by the fact that spongy platinum, 

 and other substances in a similar state, have such an affinity for oxygen gas, 

 that their capillary attraction produces the absorption of that gas from the at- 

 mospheric air into their pores, in which it is sometimes collected even in a 

 condensed state. It is probable that spongy platinum contains within its pores 

 a considerable quantity of condensed oxygen gas. Charcoal is known to ab- 

 sorb by its capillary attraction nine times and a quarter its own bulk of oxygen ; 

 and, when placed in contact with hydrogen gas, the oxygen absorbed combines 

 with the hydrogen, and forms water. The jet of hydrogen gas projected on a 



* Thomson on Heat, p. 311. 



