46 CARBONIC ACID. — EVIDENCE OF UNITY Or DESIGN. 



served to thrive better when llie quantify of this gas in the air is con- 

 siderably augmented. Common air, as lias been already stated, does 

 not contain more on an average thangTjjo^h of its bulk of carbonic acid, 

 but De Saussure found that ])lants in the sunshine grew better when it 

 was increased to y^-th of the bulk of the air, but beyond this quantity 

 they were injured by its presence, even when exposed to the sun. 

 When the carbonic acid amounted to one-half, the plants died in seven 

 days ; when it reached two-thirds of the bulk of the air, they ceased to 

 grow altogether. In the shade any increase of carbonic acid beyond 

 that which naturally exists in the atmosphere of our globe, was found 

 to be injurious. 



These circumstances it is of importance to remember. Did the sun 

 always shine on every part of the earth's surface, the quantity of carbo- 

 nic acid in the atmosphere might probably have been increased with ad- 

 vantage to vegetation. But every such increase would have rendered 

 the air less fit for the respiration of existing races of animals. Thus 

 we see that not only the nature of living beings, both y)lants and ani- 

 mals, but also the periodical absence of the sun's rays, have been taken 

 into account in the present arrangement of things. 



In perpetual sunshine plants would flourish tnore luxuriantly in air 

 containing more carbonic acid, but they would droop and die in the 

 shade. This is one of those proofs of unity of design which occasion- 

 ally force themselves upon our attention in every department of nature, 

 and compel us to recognise the regulating superintendence of one mind. 

 The same hand which mingled the ingredients of the atmosphere, also 

 set the sun to rule the day only, — tempering the amount of carbonic 

 acid to the time of his periodical presence, as well as to the nature of 

 animal and vegetable life. 



Carbonic acid consists of one equivalent of carbon and two of oxygen, 

 and is represented by CO3. It unites with bases (potash, soda, lime, 

 &c.), and forms compounds known by the name of carbonate. Thus 

 •pearlash is an impure carbonates of potash, — the common soda of the 

 shops, corbonate of soda, — and limestone or chalk, carbonates of lime. 

 From these compounds it may be readily disengaged by pouring upon 

 them diluted muriatic or sulphuric acids. From limestone it is also 

 readily expelled by heat, as in the common lime-kilns. During this 

 process the limestone loses nearly 44 per cent, of its weight, [43-7 when 

 pure and dry,] a loss which represerjts the quantity ofcarbonic acid dri- 

 ven off. [Hence by burning limestone on the spot where it is quarried, 

 nearly one-half of the cost of transport is saved,] 



Common carbonate of lime, in its various forms of chalk, hard lime 

 stone, or marble, is nearly insoluble in water, but it dissolves readily in 

 water containing carbonic acid. Thus, if a current of this gas be pass- 

 ed through lime-water, the liquid speedily becomes milky from the 

 formation and precipitation of carbonate of lime, but after a short time 

 the cloudiness disappears, and the whole of the lime is re-dissolved. 

 The application of heat to this clear solution expels the excess ofcar- 

 bonic acid, and causes the carbonate of lime again to fall. 



By exposure to the air, we have already seen that water always ab- 

 sorbs a quantity of carbonic acid from the atmosphere. As it after- 

 wards trickles through the rocks or through soil containing lime, it grad- 



