BEAT OF FLAME. ] 



UNDULATORY FORCES. LIGHT. 



91 



Fig. M. 



substances produce a similar effect when they are sifted 

 into the flame of hydrogen ; and the pyrotechnist relies 

 upon these properties in producing the various coloured 

 devices which are the glory of his art. Some years ago 

 there was an exhibition in London of philosophical fire- 

 works ; and it was described at the time as a very great 

 novehy. It was said to be effected by the combustion 

 of gases charged with various substances, and ignited 

 from jets arranged in different forms, as spirals, wheels, 

 stars, <tc. , the tints being chiefly red, blue, green, and 

 purple. It is not known what were the materials used ; 

 but from the strong smell of ether and spirit in the 

 room, there is no doubt that these liquids were among 

 the constituents of the gases, and that they were em- 

 ployed as the solvents of different saline compounds ; in 

 fact, the chromatic cloud which used to attract so much 

 attention a few years since, was nothing more than the 

 burning vapour of spirit charged with the above-named 



Klit-i. 



The Heat of Flame. Under ordinary circumstances, 

 the combustion of gaseous matter 

 takes place only at the exterior 

 of a flame that is, where the sur- 

 face of the luminous cone is in 

 immediate contact with the atmo- 

 sphere ; but we may increase these 

 points of contact, and therefore 

 the combustion and heat, by throw- 

 ing air into the interior of the 

 flame, or by mixing the combusti- 

 ble with the atmosphere before 

 it is burnt. In this way we can 

 increase the heat of flame to almost 

 any extent ; indeed, the principle 

 of the common blowpipe, of which 

 there are numerous varieties, is 

 founded on this fact ; and if we 

 blow a jet of oxygen into a flame 

 of coal gas by means of a double 

 tube, we obtain a temperature that 

 is sufficiently high to melt pla- 

 tinum. A mixture of air and 

 coal gas may also be burnt over a 

 chimney covered with wire gauze, 

 and then the heat produced is 

 sufficient to keep a small crucible 

 at a bright red-heat (Fig. 83) ; but 

 the highest temperature of all is 

 generated by the combustion of a 

 mixture of two parts, by measure, of hydrogen gas, and 

 one of oxygen. This constitutes the mixture of the 

 oxy-hydrogen blowpipe, which has been so useful an in- 

 strument in the hands of the chemist. Different com- 

 bustible substances do not, however, produce the same 

 amount of heat, or consume the same relative proportion 

 of oxygen. This is an important fact, and it should 

 always be remembered in considering the effect produced 

 by different illuminating agents in the atmosphere of the 

 room in which they are burnt. The following table 

 exhibits, in approximate numbers, the relative values of 

 different combustibles in this respect : 



Coal gas U chiefly composed of hydrogen and light 



10 '1 cubic feet. 

 8-3 

 8-3 

 6-4 

 50 

 4-0 

 3-0 



2-6 

 23 



carburetted hydrogen, with which there is a small pro- 

 portion of defiant gas ; it therefore evolves enough heat 

 to raise about forty thousand times its weight of water 

 one degree, and it produces about twice its weight of 

 carbonic acid. 



In order to show the advantage of well-purified coal 

 gas over all other materials for artificial illumination, as 

 regards the injury done to the atmosphere of the room 

 in which their combustion is going on, Dr. Frankland 

 has given the following table, which exhibits the amount 

 of carbonic acid produced by a number of illuminating 

 agents, burnt in such quantity as to give a light, for ten 

 hours, equal to that of twenty sperm caudles, each con- 

 suming 120 grains per hour : 



Tallow 



Wax 



Spermaceti 



Sperm oil 



Common London gaa . 



Manchester gas .... 



London cannel gas 



Hydro-carbon gas, with Boghead 1 

 coal gas . . . . / 



Hydro-carbon gas, with Lesma- 1 

 hagp coal gas / 



The Product* of Combustion. One of these namely, 

 carbonic acid has been noticed in the preceding table ; 

 the other product of combustion is water. Occasionally 

 there is also produced a small quantity of sulphurous 

 acid; but, as a rule, it will be found that combustible 

 substances are composed of only carbon, hydrogen, and 

 oxygen. Coal gas, however, contains a small proportion 

 of bisulphuret of carbon, and sometimes a very percep- 

 tible quantity of sulphuretted hydrogen, both of which, 

 in burning, produce sulphurous acid the acid of the 

 burning match; and this, by further oxydation of the 

 air, becomes sulphuric acid, or oil of vitriol, a product 

 which has been found to exert the most destructive in- 

 fluence on textile fabrics, as the covers of books, &<x 

 Again, when coal gas is not perfectly consumed, it gene 

 rate* aldehyde, volatile oil, and a few other compounds 

 which are exceedingly offensive.* This is experienced 

 in every kind of gas-stove, where the flame is allowed to 

 play on a large cooling or radiating surface ; and hence 

 it is advisable that gaa should always be burnt in such a 

 manner that the products of combustion may easily 

 escape into the external atmosphere. Other illuminating 

 agents, as tallow, oil, turpentine, &c. , likewise give rise 

 to the formation of unpleasant compounds when they 

 are burnt at a low temperature. Every one must be 

 acquainted with the odour of a smouldering candle or a 

 badly-trimmed lamp ; and when alcohol, ether, or wood- 

 spirit are burnt at the temperature of glowing platinum, 

 they produce a number of most irritating compounds, as 

 aldehyde, acetic acid, formic acid, <tc. This teaches us 

 that combustion should at all times be kept up with as 

 much energy as is compatible with the required effect, 

 and that the product of its action should be disposed of 

 as speedily as possible. 



Effect of Cooling Influence* on Flame. Every circum- 

 stance that tend* to lower the temperature of flame, 

 operates to a like extent in diminishing the intensity of 

 its light. We see this when a large "snuff" collects on 

 the wick of a tallow-candle the carbon of which radiates 

 the heat so fast, that the temperature and light of the 

 flame are considerably reduced ; and by bringing a mass 

 of metal in contact with a small flame, the hitter is im- 

 mediately crtinguished. Sir Humphry Davy was the 

 first to investigate facts of this description ; and he 

 showed, by his masterly researches, that flame cannot 

 exist below a certain temperature. This is easily proved 

 by intersecting the flam* of a candle with a piece of wire 

 gauze, having about thirty or forty meshes to the inch, 

 by endeavouring to pass the flame through a ring of 

 stout copper wire, or through a small hole punched in a 

 sheet of that metal. In all these cases the flame will be 

 extinguished above that point where the metal intersects 

 it ; and, although the inflammable vapour will continue 

 See ante, p. 33; artusle, "Het. M 



