1357. The quantity of carbonic acid gas in the atmosphere is very small. It is not easy to determine it with precision, and it varies slightly in different situations; but, where there is a free circulation of air, it is probably never more than one 1000th, nor less than one 2000th, of the volume of air. Carbonic acid gas is nearly one-third heavier than the other elastic parts of the atmosphere in their mixed state; hence, at first view, it might be supposed that it would be most abundant in the lower regions of the atmosphere; but, unless it has been immediately produced at the surface of the earth by some chemical process, this does not seem to be the case; elastic fluids of different specific gravities have a tendency to equable mixture by a species of attraction, and the different parts of the atmosphere are constantly agitated and blended together by winds or other causes. De Saussure found lime-water precipitated on Mont Blanc, the highest point of land in Europe; and carbonic acid gas has been always found, apparently in due proportion, in the air brought down from great heights in the atmosphere by aeronautic adventurers.