;54 



NATURE 



[August 8, 1907 



hydrocarbons at temperatures extending from those of 

 incipient oxidation up to the highest ones that prevail in 

 a (lame. According to Prof. Bone, the oxidation of a 

 hydrocarbon involves nothing in the nature of a selective 

 or preferential oxidation of the carbon or the hydrogen ; 

 but it occurs in several well-defined stages, during which 

 oxygen enters into and is incorpiorared with the hydro- 

 carbon molecule, forming o.xygcnated intermediate pro- 

 ducts, among which are alcohols and aldehydes. The re- 

 action, just referred to, between ethylene and an equal 

 volume of o.xygen is, according to Prof. Bone, to be re- 

 presented by the scheme : 



CH., CM. on CH.OII H..C:0 H.. + CO 



CH., 



CH„ 



CH.OII H.,C:0 H,+ CO 



There can be no question about the facts on which this 

 ■scheme is based, and they are a new and important 

 addition to knowledge. 



It is a great aid to the study of chemical changes, when 

 we can resolve them into stages, whether or not these 

 stages be realisable under certain experimental conditions. 

 In this way we can get a clear view of the relationship 

 between the action in one set of circumstances to the 

 action in another set ; and in this way also we can 

 often establish rational links between reactions which at 

 first sight seem quite disconnected. Intermediate reactions 

 are much used to elucidate cases of contact action, and in 

 the processes of orgam'c chemistry they are almost 

 universally assumed. 



I am far from wishing to disparage these practices, but 

 I think it important that we should realise how far we 

 are dealing with convenient devices and how far with 

 ascertained facts. The isolation of an intermediate pro- 

 duct in one set of circumstances is in itself no proof 

 that this product is transitorily formed when the reaction 

 is proceeding in another set of circumstances ; and if we 

 were to assume generally that because we can represent 

 a chemical transaction as if it were due to a successive 

 construction and destruction of a series of molecular 

 edifices it actually does take such a course, we should, 

 I think, be making the same kind of mistake as to sup- 

 pose that in the application of two differently directed 

 forces to a body at rest, the body will move successively 

 in the direction of each force instead of moving immedi- 

 ately in the direction of their resultant. I know that I 

 may be considered hvncrcritical, and perhaps obstinate, in 

 this matter ; but I wished to state the reasons that prevent 

 me from accepting entirely the interpretation which Prof. 

 Bone has given to his experimental results, and to direct 

 attention to a question of general importance that has 

 not, I think, received the attention it deserves. 



The mode of burning of carbon, whether in the free 

 state or as a constituent of a compound, is not at all easy 

 to determine; and notwithstanding many investigations, 

 among which must be specially mentioned those of Prof. 

 H. B. Dixon and his collaborators, so simple-looking a 

 question as whether carbon forms carbon monoxide by 

 directly uniting with oxygen, or only by reducing carbon 

 dioxide, is still a matter of uncertainty. 



Our knowledge concerning the question of flame 

 temperatures has been much improved in recent times, 

 thanks mainly to the admirable work of M. Le Chatelier. 

 The well-known memoir of Mallard and Le Chatelier on 

 the explosion of gases supplied the data which first per- 

 mitted of a moderately exact calculation of flame tempera- 

 tures, and the perfection of the thermo-couple by M. 

 Le Chatelier gave us the first instrument that could be 

 used directly for making a satisfactory measurement. The 

 uncertainty connected with this subject may be well illus- 

 trated by quoting the temperatures that have at different 

 times been ascribed to the flame of coal-gas when burnt 

 in a Bunsen burner, where we have had values varying 

 from 1230° to 2350° C. 



The question of calculating the temperature attained 

 during combustion by reference to calorimetric values, 

 specific heat, dissociation, and other considerations is to 

 forty] the subject of a joint discussion with Section G 

 during the present meeting, so that I shall not here enlarge 

 upon it. 



With regard to the use of thermo-couples, I may re- 

 NO. 197 I, VOL. 76] 



mark that the practical difliculties have been successfully 

 met. The chief difilculty is, of course, to secure that the 

 thermo-junction attains as nearly as possible the tempera- 

 ture of the region in which it is immersed. As ordinary 

 flames consist of thin shells of burning gases, on either 

 side of which there is a very rapid fall of temperature, it 

 is necessary to use thin wires, and to dispose them so 

 that there is no appreciable drain of heat from the junc- 

 tion. By using wires of different gauge for the couples 

 it is possible by extrapolation to arrive at a temperature 

 for a couple of infinitely small cross-section, and it is 

 also possible to [nake a correction for the superior 

 radiating power of the couple as compared with the flame- 

 gases. Without this last correction a maximum tempera- 

 ture of 1770° was obtained for the Bunsen flame by 

 Waggoner in Germany, and 1780° by White and Traver 

 in America. Correcting for radiation, Berkenbusch found 

 1830° as the maximum temperature. 



M. K^ry, by an ingenious application of his beautiful 

 optical pyrometer to a flame containing sodium, gives 

 1871° as the highest temperature of the flame of a Bunsen 

 burner burning coal-gas. 



1 he consideration of flame-temperatures has become of 

 increasing importance in the arts owing to the use of the 

 Welsbach mantle as a means of deriving light from coal- 

 gas. The great improvements which have been made in 

 the efliciency of atmospheric burners depend primarily on 

 the fact that the smaller the external surface we can 

 give to a flame consuming gas at a fixed rate the higher 

 must be the average temperature ; and since the emission 

 of light from a mantle is proportional to a high power 

 of the absolute temperature, a small increase of tempera- 

 ture is of great effect on luminosity. 



The acetylene-oxygen flame in which a temperature of 

 about 3500' prevails, not very different from that of the 

 electric arc, is the hottest of the hydrocarbon flames, and 

 finds some important practical uses. 



I have already said something about the luminosity of 

 flames so far as relates to the separation and glow of 

 solid carbon. But there remains the more general ques- 

 tion of the luminosity of flames containing nothing but 

 gases. The older explanation of the emission of light 

 from combining gases said no more than that the energy 

 liberated during the reaction and appearing as heat raised 

 the product to incandescence — that is to say, so increased 

 the velocitv of its molecules and the violence of their 

 collisions that vibratioj'.s were set up the wave-lengths of 

 whicfi lay within the limits of visible radiation. This 

 explanation has long been questioned, and there is now, 

 I think, a very general agreement that it will not suffice. 

 The average temperature, in fact, prevailing in a flame, 

 if attained in the product of combustion by the supply of 

 heat from outside, does not suffice to make that substance 

 luminous. We are therefore thrown back upon the con- 

 clusion that the generation of light in a flame is not a 

 consequence, though it is an accompaniment, of the 

 elevation of temperature. The question now is. Can we 

 go any further? To do this we are led to consider in- 

 dividual molecular transactions instead of statistical 

 averages, and the view presents itself that the combining 

 atoms may, in losing their chemical energy, form directly 

 systems of independent vibration where the radiation is 

 such as to fall within the limits of visibility. If we 

 picture such vibrating systems momentarily formed, it is 

 easy to see that by their collision one with another they 

 may acquire in a secondary way increased translational 

 motion, and so lead to a state of things where the greater 

 part of their energy is degraded in the form of heat. 

 The high temperature of a flame would then be a con- 

 sequence rather than a cause of its light. 



This subject of the mechanism of luminosity, however, 

 like so many others, has now become involved with the 

 theorv of electrons, and a chemist may be excused if he 

 hesitates to pursue the subject further. Some years ago 

 I called attention to the scantiness of our knowledge of 

 the chemical changes that take place when metallic salts 

 are used in flames for the production of spectra. Though 

 there was general agreement that, for example, the yellow 

 flame produced by common salt was due to the liberation 

 and glow of metallic sodium, there was no agreement as 

 to how the sodium was set free. 



