THE HYGIENIC INFLUENCE OF PLANTS. 419 



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. "Wolff htigel, at Munich. The result 

 is, in the main, that the variations very small from the hrst 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 preva- 

 lent 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, Wolffhiigel 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 



