Page:Popular Science Monthly Volume 12.djvu/435

Rh very recent times, a great number of experiments have been made on the subject by Roscoe in Manchester, Schulze at Rostock, and myself and my pupils, particularly Dr. Wolffhügel, at Munich. The result is, in the main, that the variations—very small from the first—have been found to be still smaller as the methods of determining carbonic acid have been perfected.

Saussure, who worked by a method liable to give an excess, found from 3.7 to 6.2 parts in 10,000. He considered that there were also slight variations between summer and winter, day and night, town and country, land and sea, mountains and valleys, which might be ascribed to vegetation. Boussingault, however, found the carbonic acid in the air to be rather less, and the same on an average in Paris and St.-Cloud: in Paris 4.13 and at St.-Cloud 4.14 in 10,000, which surprised him the more as he had reckoned that in Paris at least 2,944,000,000 litres of carbonic acid were exhaled by men, animals, and fuel.

Roscoe made experiments on the air at a station in the middle of Manchester, and at two stations in the country. He was originally of opinion that the vast manufactures of Manchester, chiefly dependent on the consumption of coal, must produce a perceptible effect on the carbonic acid in the air; but he also discovered that the air in the space in front of Owens College contained no more than the air at the country stations. He also observed occasional variations: but, when the carbonic acid increased or diminished in the city, it was generally just the same in the country. Roscoe found the greatest amount of carbonic acid in the air during one of the thick fogs prevalent in England.

Schulze found the amount of carbonic acid in the air at Rostock to be between two and half and four parts in 10,000. On an average it was somewhat higher when the wind blew off-shore than off the sea.

In Munich, Wolffhügel found the carbonic acid to be between three and four parts in 10,000. Now and then, but very seldom, he observed variations, the maximum being 6.9 parts in 10,000 in a very thick fog, the minimum 1.5 part in 10,000 in a heavy snow-storm, when the mercury was very low in the barometer.

It may be asked how the immense production of carbonic acid in a city like Paris or Manchester can thus vanish in the air. The answer is very simple: by rarefaction in the currents of the atmosphere. We are apt not to take this factor into account, but think rather of the air as stagnant. The average velocity of the air with us is three metres per second, and even in apparently absolute calm it is more than half a metre. If we therefore assume a column of air 100 feet high and of average velocity, it may be reckoned that the carbonic acid from all the lungs and chimneys of Paris or Manchester is not sufficient to increase its amount so as to be detected by our methods.

From this fact it may be logically concluded that, if no increase