THE DUST OF THE AIR 231 



tion with less efficiently ventilated rooms, is the immense 

 increase in the number of dust-particles after the gas is 

 lighted of an evening. More sensational, perhaps, is the 

 fact that a cigarette smoker sends 4,000,000,000 particles, 

 more or less, into the air with every puff. 



Of great biological importance is the contrast between 

 the freshness of the country and the dustiness of towns. 

 One clear day at Colmonell in Ayrshire there were only 

 500 particles (about the minimum) per cubic centimetre, 

 or 8200 per cubic inch, while on a wet morning near the 

 Central Station in Glasgow there were 466,000 per cubic 

 centimetre, or 7,642,400 per cubic inch. The brilliancy of 

 Swiss air is due to the comparative scarcity of dust, while 

 the dismal fogs of our large cities are due to its abund- 

 ance. The more dust-particles, in short, the murkier the 

 air, and what we call haze is often little more than dust. 

 The botanists tell us of plants which are particularly 

 sensitive to the dust in the air, and fade away when it 

 passes their limit of viability. The beautiful red Tropaeolum, 

 the summer ornament of so many cottages in the country, is 

 one of these dust-sensitive plants. Another fact of obvious 

 biological interest is the purifying effect of the rain. Aitken 

 and his followers have shown, for instance, how a heavy 

 shower may clean for a brief period the dust-laden atmo- 

 sphere of a town. A gale blowing from a pure quarter has 

 the same clarifying result. 



It is characteristic of modern science that it appreciates 

 the cumulative importance of infinitesimally small items. 

 The commonplace that "many littles make a mickle" is a 

 generalisation which expresses some of the grandest results 

 of patient observation. So beside bacteria and earthworms, 

 beside coral polyps and chalk-forming animalcules, we must 

 rank dust as an agency of high importance. Whether it 

 be borne on the wings of the wind from one land-surface 



