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 considerable 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 observation made. Though at the sea-level the wind was steady, and the thermometer did not vary, at the summit the wind suddenly veered round to the opposite direction of that below, blowing 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 transformed 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
