CARBONIC ACID. 367 



72 oxygen, and all the results come so near to this, that we may 

 venture to neglect the fractions. The composition of carbonic acid 

 is a problem of great importance, for whenever it is produced, we 

 infer the presence of carbon in the proportion now stated. 



(i.) Oxygen gas, by uniting with charcoal, suffers neither contrac- 

 tion nor expansion, but increases- in specific gravity, so that 100 

 cubic inches weigh, at the medium temperature and pressure, 46.59 

 grains, or about one and a half the weight of common air.* 



These methods of obtaining carbonic acid gas, are put in practice 

 only to demonstrate its composition ; they are never resorted to when 

 the object is to obtain the gas in large quantities ; then it is always 

 extracted from some of its natural combinations. 



2. OTHER MODES OF OBTAINING CARBONIC ACID GAS. 



(a.) Procured from marble poivder, or chalk with dilute sulphuric 

 or muriatic acid.^ The proportions with sulphuric acid, may be 

 about 6 parts by weight, of water, to 1 acid, and 1 J marble powder ; 

 apparatus a retort, flask, or bottle, with a glass tube, bent twice at 

 right angles, and turned up at the end of delivery ; it may be thrust 

 through a cork bored by a tapering hot iron ; the residuum will be 

 sulphate of lime. 



(b.) Heat marble powder or chalk, red hot, in an iron bottle; a 

 quart affords a barrel of gas, and the residuum is brought almost to 

 the condition of quick lime. 



3. DECOMPOSITION. 



(a.) Decomposed by repeated electrical discharges, over mercury; 

 becomes carbonous oxide, J and oxygen gas. 



The undecomposed carbonic acid, being washed out by lime wa- 

 ter, or potassa, and an electric discharge passed through the remain- 

 der, it explodes and becomes again carbonic acid. 



(b.) A mixture of hydrogen and carbonic acid, being heated in 

 the same manner, water and oxide of carbon are obtained. 



(c.) Carbonic acid, as it exists in the carbonate of lime, and of 

 baryta, and probably strontia, is easily decomposed by igniting the 

 pulverized carbonate with iron filings, when oxide of carbon is pro- 

 duced, as will be shewn in connexion with that substance. 



(d.) Potassium heated in carbonic acid gas, in the proportion of 

 5 grains to 3 cubic inches, inflames, and charcoal is precipitated. || 



* For the statements of different writers, see Henry. 



t Muriatic acid, mixed with 2 or 3 parts of water, is perhaps preferable, be- 

 cause the sulphuric acid forms an insoluble compound with the lime, and clogs the 

 effervescence. 



t Whose propertes will be soon explained. 



|| I am accustomed to exhibit this beautiful experiment by the following arrange- 

 ment. A flask, with dilute sulphuric acid and marble powder, is fitted with a cork and 

 tube bent twice at right angles, through which carbonic acid gas flows to the bottom of 

 another flask, and expels the air, or the gas may be introduced in a similar manner 



