364 



THE POPULAE- EDUCATOR. 



vapours as they rise from the wick. The hydrogen, having a 

 much greater affinity for the oxygen than the carbon has, at 

 once appropriates it all, forming water ; but, as we have before 

 noticed, the combination of oxygen and hydrogen produces 

 great heat. This raises the atom of carbon, just liberated from 

 its combination with the hydrogen, to a white heat. This 

 process goes on in the light-giving cone of the flame, i. These 

 heated particles of carbon pass to the outside of the flame, where 



they combine 

 with the oxy- 

 gen of the air, 

 first forming 

 carbonic oxide, 

 which again, 

 taking another 



? 40. JH^. S9> \ atom of oxy- 



gen, burns into 

 carbonic acid 

 with a blue 

 flame, e, which 

 forms the 



third cone of the flame. Thus it is seen that the luminosity 

 of a flame entirely depends upon the quantity of air which can 

 find an entrance into the body of the vapour undergoing com- 

 bustion. If there be present too little oxygen, some of the 

 hydrogen will pass to the outside of the flame unburnt, and 

 thus much of its heating power will be wasted. On the other 

 hand, if too much oxygen mix with the flame, then not only is 

 the hydrogen burnt, but also some of the carbon at once 

 becomes carbonic oxide, and the illuminating power of the 

 flame will be diminished. This is illustrated in "Bunsen's 

 burner." 



Here the air is mixed with the gas before it escapes at the 

 top of the tube. When ignited, there is not only sufficient 

 oxygen to satisfy the demands of the hydrogen, but also enough 

 for the combustion of the carbon, which is never " free," burn- 

 ing at once into carbonic oxide. If the holes be covered with 

 the fingers, the flame becomes luminous, the supply of air 

 being limited to that which can mix with the gas as it escapes 

 from the tube. A flame may be deprived of its luminosity by 

 simply blowing it, for then, by mechanical means, sufficient 

 oxygen has been introduced to burn the carbon, and the flame 

 becomes blue. 



It will be evident, then, in all flames, that the maximum 

 illuminating power will entirely depend on the right quantity 

 of air ; as may often be noticed by moving the chimney of a 

 moderator lamp higher or lower. 



If a cold surface be depressed into the flame of a Bunsen's 

 burner or a spirit-lamp, it will be found that a film of moisture 

 is deposited upon it, which is, of course, driven off as soon 

 as the body is heated, for the products of such a flame are 

 water and carbonic acid, and the latter being a gas cannot be 



deposited ; but 

 if the same ex- 

 periment be 

 _ tried in a lumi- 



sides the .mois- 

 ture, carbon 

 soot will be 

 found on the 

 cold surface ; 

 the reason 

 being, that car- 

 bon will only 



combine with oxygen at certain temperatures. Coals will remain 

 in a cellar for any length of time, but when once heated to a cer- 

 tain temperature their combination with oxygen commences, and 

 we say they burn. In a flame, the combustion of the hydrogen 

 just produces a sufficient heat to raise the carbon to this 

 required temperature; but if any of this heat be detracted 

 by the cold surface, for instance, above alluded to the carbon 

 is deprived of the power of combustion, and is therefore depo- 

 sited in its elementary condition. 



This accounts for the smoking of fires. Cold air passes 

 through them, and its temperature is raised at the expense of 

 the heat given out by the combustion of the hydrogen, thus 

 taking away the heat necessary for the complete combustion of 



Pig. 41. 



the carbon, a portion of which comes away unburnt in the shape 

 of smoke. If a fire can be fed with hot air, or supplied with 

 a limited quantity, all the carbon will be burnt, and there will 

 be no smoke. One or other of these means is resorted to in 

 all "engine" fires. Perhaps the most satisfactory method is 

 to bend an iron pipe, about four inches in diameter, in the form of 

 a U : place it beneath the grate, and turn one end into the mouth 

 of the furnace, which is closed by tightly-fitting doors. The 

 air passes through the tube, which is red-hot, and then enters 

 the furnace, which is now enabled to burn without smoke. 



A flame cannot exist if too much heat be taken from it. 

 Thus, when we blow a candle too violently, it "goes out," 

 because we have caused such a quantity of cold air to take 

 away its heat as to render it unable to exist. 



If a piece of wire gauze be held in the flame of a spirit-lamp 

 (Fig. 40), the flame will not pass through the gauze, although 

 the unburnt vapour will, which may be ignited on the upper 

 side. The fact is admirably 

 shown in Fig. 41, where the 

 gas from a Bunsen's burner is 

 lit above the gauze, but is un- 

 able to ignite the gas below 

 it, because at the place where 

 the flame touches the gauze 

 it is extinguished. The iron, 

 which is a good conductor of 

 heat, robs the flame of so 

 much, that it cannot exist, and 

 is therefore extinguished. 



The Davy lamp owes its 

 value to this fact. Fig. 42a 

 shows that the common oil- 

 lamp, b, is supported by a 

 frame of iron wire, while 

 the flame is entirely enclosed 

 in a cylinder of iron wire 

 gauze, d. ~Fig. 42. 



The " fire-damp " passes 



through the gauze, and is ignited by the flame. An explosion 

 occurs in the lamp, which extinguishes the light ; but the cool- 

 ing power of the gauze prohibits the flame from passing out to 

 ignite the mine. Thus the miner is warned of his danger. 

 Even if the flame of the lamp be allowed to come in contact 

 with the cylinder, it cannot do more than make it red-hot, and 

 as fire-damp cannot be ignited unless by a white heat, there is 

 no danger. 



The cylinder is fastened down to the body of the lamp by a 

 lock, the key of which is kept by the clerk of the mine. The 

 miner trims his wick by the wire c, which passes through the 

 oil in a tube, as the section shows. 



The gauze cylinder greatly darkens the light, and the miner 

 is much tempted to work with a naked flame ; and to this 

 recklessness may be traced almost all colliery explosions. 



The results of the combustion of fire-damp are water, which 

 condenses on the sides of the mine, and carbonic acid choke- 

 damp. 



LESSONS IN FRENCH. XXXIX. 



SECTION LXXXVIL IDIOMS RELATING TO PRONOUNS, 



ETC. 



1. THE pronouns subject, je, tu, il, elle, nous, vous, ils, elles, 

 must be repeated, when the first verb of the sentence is nega- 

 tive, and the second affirmative, when the verbs are in different 

 tenses, and when the different clauses are connected by con- 

 junctions other than et, ou, ni, mais [ 96, 2] : 

 II ne lit pas ; il ecrit. He does not read; he writes. 



Elle lie viendra pas ; elle est She Kill not come ; she is gone. 



partie. 



2. The pronouns of the third person are often omitted before 

 the second verb in cases not coming within the above rule. 

 The other nominative pronouns are also, sometimes, omitted. 

 We should, however, not advise the student to omit the latter 

 pronouns. It is always correct to repeat the nominative pro- 

 nouns. 



3. The student will bear in mind that the objective pronouns 

 must always be repeated. 



4. Connaitre a answers to the English expression, to know by : 



Je le conuais a sa demarche. 



him by hie walk (carriage). 



