October 26, 1893] 



NATURE 



627 



in order to make sure that the action was completed, he allowed 

 the whole apparatus to gradually cool down again to its original 

 temperature. 



Before starting the experiment he had carefully measured the 

 air in the apparatus, which amounted to fifty cubic inches, and 

 the first thing which he now noticed was that of this forty-two 

 cubic inches only remained, and that this residual gas had lost 

 all the most characteristic properties of air ; a taper plunged 

 into it was at once extinguished, a mouse placed in it died after 

 a few moments ; it would, in fact, neither support life nor com- 

 bustion, and he recognised it as a gas discovered some three 

 years before by Rutherford, and now called nitrogen. 



He then collected the red film formed on the surface of the 

 mercury, which weighed forty-five grains, and heated the 

 powder in a hard glass tube to a higher temperature than that 

 at which it had been formed, when it again broke up, leaving 

 behind metallic mercury, and yielding eight cubic inches of a 

 gas which had to an exaggerated extent all the properties which 

 the air had lost — a gas which he at once recognised as being 

 the oxygen or "vital" air which Priestley had discovered in 



1774- 



It was in this way that the air was shown to consist of the 

 two gases, oxygen and nitrogen, and we know from experience 

 that air is necessary for carrying on those cases of combustion 

 which we ordinarily meet with, and the quickest way to ex- 

 tinguish a fire is to cut off the supply of air from it. 



Having reached this point, the next question which suggests 

 itself is, which of the constituents of the atmosphere is it which 

 supports and carries on combustion, and how does it act in 

 doing so ? And the answer to these points can most readily be 

 given in Nature's own words, by carefully translating the result 

 of a few simple experiments. 



Here are two gas jars, the one containing oxygen, the other 

 nitrogen, and, taking a small ball of tow soaked with turpen- 

 tine which is burning vigorously, I plunge it into the atmosphere 

 of nitrogen, when it is at once extinguished, but on now re- 

 lighting it, and plunging it into the oxygen, it burns far more 

 fiercely than before, and emits a most brilliant light. If we 

 continued experimenting in this way, we should find that every- 

 thing tends to confirm the impression gained from our first ex- 

 periment, and we soon learn, as Lavoisier did, that anything 

 which will burn in air will burn with still greater vigour in 

 oxygen, whilst nitrogen alone instantly stops the combustion of 

 those bodies which require air to enable them to burn ; indeed, 

 we might go a step beyond Lavoisier's experiments, and find 

 that many bodies not looked upon as combustibles, such as iron 

 and zinc, burn with considerable brilliancy in pure oxygen ; and 

 it i» from these facts that we came to look upon oxygen as our 

 great supporter of combustion. 



The enunciation of these truths by the great French philo- 

 sopher was one of the most important steps in the history of 

 science, but with increase of knowledge we find that we must 

 still further widen our views with regard to combustion, and 

 must take care not to fall into the error of looking upon those 

 substances which will burn in air or oxygen as the only com- 

 bustibles, and oxygen as the only supporter of combustion ; we 

 find, indeed, that these terms are purely relative, and a substance 

 which we look upon as a combustible may, under altered condi- 

 tions, become a supporter of combustion. Indeed, a body like 

 coal gas, which burns in air or oxygen, will support in turn the 

 combustion of air, and we can experimentally show that it is 

 just as easy to have a flame of air burning in coal gas, as under 

 ordinary conditions to have a flame of coal gas burning in air. 



Again, we find that many cases of combustion will take place 

 without the presence of oxygen or those substances generally 

 looked upon as combustibles, and we can take a metal like 

 antimony, and cause it to undergo brilliant combustion by 

 throwing it in a powdered condition into an atmosphere of 

 a gas called chlorine, although neither the metal nor the gas 

 answer to our general ideas as to combustible or supporter of 

 combustion. 



If we examine carefully all cases of combustion, we find that 

 in them we have a body with certain definite properties of its 

 own, uniting itself with something else to form what we call the 

 products of combustion, which are equal in weight to the sum of 

 the weights of the two bodies uniting, and which have charac- 

 teristic properties differing from those of the original substances, 

 an action which we term one of chemical combination ; and ex- 

 tended experiments show us that in order to obtain a true con- 



ception of combustion, we must look upon it as "the evolution 

 of heat during chemical combination." 



The rapidity with which chemical combination takes place 

 varies to a very great extent with surrounding circumstances, 

 and inasmuch as heat is very rapidly dissipated it often happens 

 that where a chemical combination is slow, the heat produced 

 by it is given off as rapidly as it is generated, so that the tem- 

 perature of the mass becomes but little raised, and escapes 

 detection 'oy our senses. For instance, if I take a steel watch- 

 spring, and having ignited a small piece of German tinder 

 attached to the end of it, plunge it into a vessel of oxygen gas, 

 the combustion of the tinder ignites the watch-spring, which 

 burns away in the gas with the greatest brilliancy, and the 

 evolution of heat is sufficient to fuse some of the metal, the 

 result being that the watch-spring is converted into a chemical 

 compound of iron and oxygen. If instead of bringing about 

 the combination of the iron and oxygen as we have done in a 

 few seconds, we allowed it to remain in moist air for two or 

 three months, combination with the oxygen of the air would 

 result, and the metal would rust away, and if the weight of 

 metal had been the same in each case, and the same weight of 

 oxygen had been combined with, exactly the same amount of 

 heat would have been generated in each case ; but in the rapid 

 combustion of the metal, this heat, being all generated in a few 

 seconds of time, would have made its presence perfectly mani- 

 fest ; whilst when the same action is spread over a long period, 

 as in the rusting of the metal, the heat being dissipated as it is 

 generated, escapes our notice ; and there are many amongst us 

 who would smile at the idea that the rusting of their garden 

 railings was giving rise to any increase of temperature. 



In this case the heat generated by the combination of the iron 

 with oxygen was made manifest by raising the burning metal to 

 a high temperature in the presence of oxygen free from the 

 diluting action of the inert nitrogen which is mixed with it in 

 the air ; but we can do the same thing by taking the iron in a 

 very finely-powdered condition, so that a very large surface 

 shall be exposed to the action of the oxygen of the air. I have 

 here iron in this condition, sealed up in a glass tube, and on 

 opening and shaking out the finely-divided metal into the air, 

 it at once enters into combination with oxygen, and the heat 

 generated is sufficient to make it red-hot. If, however, the same 

 weight of iron in a compact form, such as wire, be taken, a long 

 period of time, extending perhaps over years, would be re- 

 quired for its conversion into oxide by air and moisture, and the 

 heat generated would be spread over such a duration of time 

 that it would be inappreciable, unless the conditions were 

 such that the heat was unable to escape or the surface of 

 metal exposed very large. A case of this kind occurred 

 during the manufacture of the Mediterranean telegraph cable, 

 which was enclosed in a strong casing of iron wire, and 

 tightly coiled in water tanks, one hundred and sixty-three miles 

 ot cable being wound in a coil thirty feet in diameter. Owing 

 to a leak in the tank which contained the cable the water ran 

 off, leaving the wire casing exposed to air, and the moist metal 

 oxidised so rapidly that sufficient heat was generated to form 

 considerable quantities of vapour, and to give rise to serious 

 fears as to the softening of the insulating material of the core. 



Many cases of chemical combination with the oxygen of the 

 air take place in nature, which are so slow that the heat 

 evolved during the action escapes our senses, and indeed all 

 cases of decay are processes of this kind, and the action is 

 termed one of "slow combustion. " 



A tree left to rot upon the ground gradually disappears in the 

 course of years, being mainly oxidised into gaseous products 

 such as carbon dioxide and water vapour, and yet scarcely any 

 evolution of heat is observed, although the same amount of 

 heat is generated as if the tree had been cut into logs and 

 burnt. 



In all cases slow combustion is accelerated by increase of 

 temperature, and the higher the temperature the more rapid 

 becomes the chemical action, and all combustible bodies, at a 

 certain temperature, undergo what is termed "ignition," that 

 is to say, a temperature is reached at which slow combustion 

 passes into ordinary combustion with manifestation of flame or 

 incandescence, the chemical combination being then so rapid 

 that the heat evolved is manifest to our eyesight, whilst a still 

 greater increase in the rapidity of combustion will in some cases 

 iiring about the most rapid form of combustion, which we term 

 "explosion." 



NO. 1252, VOL. 48] 



