"236 PRINCIPLES OF CIIE3IISTKY 



amount of carbonic anhydride in the air (respiration, combustion, 

 rotting, volcanic eruptions, ifec.), or diminish it (absorption by plants 

 and water), therefore the reason of the great constancy in the amount 

 of this gas in air must be looked for, in the first place, in the fact that 

 the wind mixes the air of various localities together, and, in the second 

 place, in the fact that the waters of the ocean, holding carbonic acid in 

 solution, 31 form an immense reservoir for regulating the supply of this 

 gas in the atmosphere. As immediately the partial pressure of the 

 carbonic anhydride in the air decreases, the water evolves it, and when 

 the partial pressure increases, it absorbs it, nature consequently sup- 

 plies the conditions for a natural mobile state of equilibrium in this 

 instance, as in a number of others. 32 



Beyond nitrogen, oxygen, moisture, and carbonic acid, all other sub- 

 stances occurring in air are found in innnitesimally small quantities by 

 weight, and therefore the it-eight of a cubic measure of air depends, to a 

 sensible degree, on the above-named components alone. We have 

 already mentioned that at and 760 mm. pressure the weight of a 

 cubic litre of air is 1-293 grams, 33 the air being understood to be 



the amount of carbonic anhydride is also very slight (at a height it is, however, less, as 

 would be expected). 



51 In the sea as well as fresh water the carbonic acid occurs in two forms, directly 

 dissolved in the water, and combined with lime, as calcium bicarbonate (hard waters 

 sometimes contain very much carbonic acid in this form). If the tension of the carbonic- 

 anhydride in the first form varies with the temperature, and its amount with the partial 

 pressure, that in the form of acid salts is under the same conditions, because direct 

 experiments have shown a similar dependence in this case, although the quantitative 

 relations are different in the two cases. 



52 In studying the phenomena of nature one inevitably arrives at the idea that the 

 universally reigning state of mobile equilibrium forms the chief reason of that harmonious 

 order which impresses all observers. It not unfrequently happens that we do not see the 

 causes regulating the order and harmony ; in the particular instance of carbonic anhy- 

 dride it is a striking circumstance that in the first instance a search was made for an 

 harmonious and strict uniformity, and in incidental (insufficiently accurate and fragmen- 

 tary) observations conditions were even found for concluding it to be absent. When, later, 

 the rule of this uniformity was confirmed, then the causes regulating such order were 

 also discovered. The researches of Schloesing were of this character. Deville's idea of 

 the dissociation of the acid carbonates of sea- water is suggested in them. In much else, 

 also, a right understanding can only be looked for in detailed investigation. 



53 The difference of the weight of a litre of dry air (free from carbonic anhydride) at 

 and 760 mm., at different longitudes and altitudes, depends on the fact that the force of 

 gravity varies under these conditions, and with it the pressure of the barometrical column 

 also varies. This is treated in detail in my works On tin- Elasticity of Gcuet SJad (>>/ 

 Barometric Levellings. 



In reality the weight is not measured in absolute units of weight (in pressure refer 

 to works on mechanics and physics), but in relative units (grams, scale weights) whose 

 mass is one and the same, and therefore the variation of the weight of the weights itself 

 with the change of gravity must not be here taken into account, for the matter deals with 

 weights proportional to the masses, and with a change of locality the weight of the weights 

 varies as the weight of a given volume of air does. 



