CHEMICAL HISTORY OF A CANDLE 159 



of the nature of this combustion of the candle to find that 

 this substance issuing from the candle is exactly the same 

 as that substance which would issue from a retort if I were 

 to put some chalk into it with a little moisture and make it 

 red-hot ; you would then find that exactly the same substance 

 would issue from it as from the candle. 



But we have a better means of getting this substance, and 

 in greater quantity, so as to ascertain what its general char- 

 acters are. We find this substance in very great abundance 

 in a multitude of cases where you would least expect it 

 All limestones contain a great deal of this gas which issues 

 from the candle, and which we call carbonic acid. All 

 chalks, all shells, all corals, contain a great quantity of this 

 curious air. We find it fixed in these stones, for whicfr 

 reason Dr. Black called it " fixed air " finding it in thest 

 fixed things like marble and chalk he called it fixed air 

 because it lost its quality of air, and assumed the condition 

 of a solid body. We can easily get this air from marble. 

 Here is a jar containing a little muriatic acid, and here is 

 a taper which, if I put it into that jar, will show only the 

 presence of common air. There is, you see, pure air down 

 to the bottom; the jar is full of it. Here is a substance- 

 marble (") a very beautiful and superior marble; and if I 

 put these pieces of marble into the jar, a great boiling 

 apparently goes on. That, however, is not steam; it is a 

 gas that is rising up; and if I now search the jar by a 

 candle, I shall have exactly the same effect produced upon 

 the taper as I had from the air which issued from the end 

 of the chimney over the burning candle. It is exactly the 

 same action, and caused by the very same substance that 

 issued from the candle ; and in this way we can get carbonic 

 acid in great abundance we have already nearly filled the 

 jar. We also find that this gas is not merely contained in 

 marble. Here is a vessel in which I have put some common 

 whitening chalk which has been washed in water and de- 

 prived of its coarser particles, and so supplied to the plas- 

 terer as whitening here is a large jar containing this 

 whitening and water ; and I have her* some strong sulphuric 



M Marble is a compound of carbonic acid and lime. The muriatic acid, 

 being the stronger of the two, takes the place of the carbonic acid, which 

 escapes as a gas, the residue forming muriate o lime or chloride of calcium. 



