DUST. 255 



average in his mind. Suppose the average number upon one of 

 these squares were five, then on the whole table there would be 

 500; and these 500 mist-particles contain the 500 dust-particles 

 which floated invisibly in the cubic inch of mixed air above the 

 table. But, as there are forty cubic inches of mixed air in the 

 flask and syringe, the number of dust-particles in the whole is 40 

 times 500 = 20,000 ; that is, there are 20,000 dust-particles in the 

 small quantity of common air (one tenth of a cubic inch) which was 

 introduced for examination ; in other words, a cubic inch of that 

 air contains 200,000 dust-particles — nearly a quarter of a million. 



By this process Mr. Aitken has been able to count 7,500,000 of 

 dust-particles in one cubic inch of the ordinary air of Glasgow. 

 We counted with him 4,000,000 in a cubic inch of the air outside 

 of the Royal Society Rooms, Princes Street, Edinburgh. Inside 

 the room, after the Fellows had met for two hours, on a winter 

 evening — the fire and gas having been burning for a consider- 

 able time — we found 6,500,000 in a cubic inch of the air four feet 

 from the floor ; but near the ceiling no fewer than 57,500,000 were 

 counted in the cubic inch. He counted in one cubic inch of air 

 immediately above a Bunsen flame the fabulous number of 489,- 

 000,000 of dust-particles. The lowest number he ever counted 

 was at Lucerne, in Switzerland : 3,500 in the cubic inch. On the 

 summit of Ben Nevis the observer, using Mr. Aitken's apparatus, 

 counted from 214,400 down to 840 in the cubic inch. But on the 

 morning of the 21st of July last there was a most marvelous ob- 

 servation made. Though at the sea-level the wind was steady, 

 and the thermometer did not vary, at the summit the wind sud- 

 denly veered round to the opposite direction of that below, blow- 

 ing out of a cyclone, and the temperature rose ten degrees. In 

 consequence the extraordinarily low mean of only thirty-four 

 dust-particles to the cubic inch was observed. 



We now come to the most pleasant of the investigations in 

 connection with dust. The very brilliant sunsets which began in 

 the autumn of 1883, and continued during successive seasons with 

 gradually decreasing grandeur, have arrested the attention of the 

 physicist as well as of the general observer. What is the cause of 

 the brilliant coloring in these remarkable sunsets ? What is the 

 source of the immense wealth of the various shades of red which 

 have been so universally admired ? Gazing on a gorgeous sunset, 

 the whole western heavens glowing with roseate hues, the observer 

 sees the colors melting away before his eyes and becoming trans- 

 formed into different hues. The clouds are of different sizes and 

 of all shapes. Some float virgin-like in silver folds, others voyage 

 m golden groups ; some are embroidered with burning crimson, 

 others are like " islands all lovely in an emerald sea." And when 

 the flood of rosy light, as it deepens into bright crimson, brings 



