Chap. 8.] Het erogenous Matters in Air. 407 



a rapidity which would carry deftruction through the 

 whole expanfe of nature. 



The air of our atmofphere is, however, notfo fimple 

 a fubftance as to be formed only of two ingred/ents. 

 Befides the fmall portion of carbonic gas or fixed air 

 which it contains, equal to one hundredth part, as was 

 intimated in the beginning of this chapter, it is alfo 

 well known that a large portion of water is ufually held 

 in the atmofphere, fometimes in a ftate of perfect fo- 

 lution, or entirely invifible, and fometimes vifible in 

 the form of mifts and clouds. The atmofphere is alfo 

 the general recipient of all thofe fubftances which are 

 fubject to evaporation, and which preferve their aeri- 

 form ftate under its ordinary heat and preffure. 



From the mixed nature of the mafs, and particu- 

 larly from the mixture of carbonic acid gas or fixed air, 

 feveral effects are produced, and fome of them it may 

 be proper to notice. Fixed air being in reality an im- 

 perfect acid, contributes to ruft metals, and to change 

 the colour of fuch purple dyes as are produced from 

 vegetable fubftances. This is an effect which moft 

 perlons have noticed, though the caufe has not been 

 underftood \ and the delicate nature of thefe colours 

 has been almoft an invariable objection - aainft their 

 ufe. 



The air of the atmofphere is moft generally injured 

 by the deft ruction of the pure part, and the generation 

 of carbonic acid gas, as in moft of the procefles of 

 combuftion, and in that of refpiration. When it is ne- 

 ceffary to purify the air from the carbonic acid, which 

 may be too abundant in it, any contrivance for bringing 

 it into contact with lime-water will fufficiently anlwer 

 this purpofe. A cloth dipped in that liquor, and fuf- 

 pended near the floor, will generally purify the air of 

 a room from any contamination of'fixed air. 



D d 4 Combuftion 



