The Chemistry Of Common Things.

The third gas ivhich exists in ihe atmosphere , carbonic acid , is itself a compound , being composed of six parts of carbon and sixteen parts of oxygen . Carbon , in its pure and crystalized state is the diamond ; in its impure , and most common form , it is the substance knoAA'ii as charcoal . Its proportion in the atmosphere is generally considered to be about one part in one thousand . It foims nearly one

half of most earthy substances , and is respired from the lungs in breathing . When the air is taken into the lunge , its oxygen combines with the carbon contained in the venous blood , which , if suffered to remain would act deleteriously . This forms carbonic acid , Avhich together with the nitrogen , is expelled . Carbonic acid is also formed by the combustion of substances Avhich contain carbon . It is absorbed

by water , and is decomposed b y plants AA'hen under the influence of light , which retain the carbon , and leave the oxygen free . Thus the requisite supply of oxygen is constantly maintained , and indeed very little of the oxygen of the atmosphere appears to be altogether abstracted from it , aud this little is replaced by the evolution of oxygen from the decomposition of Avater , and volcanic eruptions .

carbonic acid gas may be obtained by pouring an acid , vinegar for example , on chalk . It is destructive to animal life , and a lighted paper plunged into it is immediately extinguished . It is so much heavier than common air , that it may be poured from one vessel to another like Avater . From this cause it exists in great abundance at the bottom of mines and caves . The Grotto del Cairo , in Naples , is

a Avell knoAvn example . The bottom of this cave is covered by a stratum of carbonic acid , and if a dog or other small animal be made to enter , it is immediately stupified by inhaling the poisonous gas ; but a man may enter with impunity , for the fluid , liaving by its superior Avei ght settled at the bottom of the cave , the air at the level of his head is left sufficiently pure for him to breath .

The best test for the presence of carbonic acid is lime Avater ; the carbonic acid uniting with the lime , forms an insoluble precipitate of carbonate of lime . A vessel containing lime Avater , Avhen exposed to the atmosphere becomes coated with a thin pellicle oi' carbonate of lime , thus shoAving the existence of the acid in the atmosphere . The remainder of the atmosphere consists of aqueous A'apour , and various vapours and effluvia floating in it .

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