THE EVENING LESSON. 157 



Btudy to-night; then you can do another half hour's to- 

 morrow morning after breakfast, before we set out for 

 Paris ; then one hour of study on the journey, and another 

 of writing, perhaps, after you get to Paris, will make up 

 your time." 



John agreed to this proposal, and then they both went 

 to a corner of the cabin, where they could sit together and 

 talk by themselves, and Lawrence commenced his lecture 

 at once. 



" The first thing that you are to put down in your reca- 

 pitulation," said Lawrence, " is that the general principle 

 on which bright artificial lights are produced is by raising 

 solid substances of some kind to an intense degree of in- 

 candescence by means of extreme heat. The heat is gen- 

 erally produced by combustion, though not always so. 

 But, however the heat may be pi'oduced, it is always by its 

 effect on the particles of a solid substance in causing them 

 to emit a very vivid light that almost all bright lights are 

 made. 



"Then, the second thing that you must say," continued 

 Lawrence, " is that, in order to produce the required inten- 

 sity of heat by ordinary combustion, the thing to be done 

 is to increase the supply of oxygen. This is effected, as 

 you know, in the case of the Argand burner, by making 

 the wick circular, and bringing a current of air up on the 

 inside of it, and also increasing the strength of the current 

 on the outside by means of a glass chimney. 



" So, after stating the two principles, namely, first, that 

 the light is produced by making solid particles intensely 

 incandescent by means of extreme heat, and, secondly, that, 

 when this heat is to be produced by combustion, it is done 

 by increasing in some way the supply of oxygen, then you 

 can enumerate the five principal modes of producing bright 

 light, and make them subordinate heads in your article, 



