500 



SCIENTIFIC NEW^S. 



[May 25, iJ 



attempted to compute the number of " the gay motfs 

 that people the sunbeam." 



The extremely fine inorganic dust which floats in the 

 air is, in ordinary circumstances, invisible to the naked 

 eye. But even after a room has been thoroughly 

 cleaned, with all a housekeeper's anxiety and care, you 

 have only to close the shutters on a sunny day, and let 

 the sun's rays enter by a narrow chink, in order to 

 observe the enormous number of particles glistening in 

 the beam. So small and numerous are these motes that 

 it seems impossible to count them with any approxima- 

 tion to accuracy. 



About a year ago it wa? proved that without dust 

 there could be no fogs, no mist, no rain. Without dust 

 there would be only dew on the grass and road. A very 

 simple experiment will illustrate this. Let common air 

 be forced through a filter of cotton-wool into a glass 

 receiver, from which the air has been exhausted. Also 

 let a glass receiver, filled with common air, be placed 

 beside it. If both receivers be connected with a boiler, 

 and steam be allowed to pass into them, the following 

 unexpected result will be seen. In the vessel con- 

 taining the ordinary dusty air, the steam, as soon 

 as it enters, will rise in a dense cloud, and this will 

 afterwards form a beautiful white cloud, so thick that 

 it cannot be seen through. But in the vessel containing 

 the filtered air — that is, in which the dust has been 

 removed by the cotton-wool — the steam is not seen at 

 all ; there is not the slightest appearance of cloudiness, 

 although it is as full of water-vapour as the other 

 receiver. 



From this experiment it is concluded that particles of 

 water-vapour do not combine with each other to form a 

 cloud particle, and that some solid body is necessary for 

 its formation. The fine particles of dust in the air act as 

 free-surfaces on which the water-vapour condenses into 

 fog. When there is an abundance of dust in the air, and 

 only a little water-vapour, the condensed vapour torms a 

 fog ; but wtien the number of particles is comparatively 

 small, each particle can take hold of a greater share of 

 the moisture, and instead of small fog-particles being 

 formed, mist-particles, and even rain-particles would be 

 produced. 



On the principle that every fog-particle has embosomed 

 in it an invisible dust-particle, a method has been 

 devised by Mr. John Aitken for counting the 

 number of particles ^of dust in a given quantity of air. 

 His plan is to dilute a definite small portion of common 

 air with a fixed large quantity of filtered air, and allow 

 the mixture to be super-saturated by water-vapour ; the 

 few particles of dust seize the moisture, become visible 

 in fine drops, fall on a divided plate, and are there 

 counted by means of a magnifying glass. 



Having made these preliminary remarks on the 

 principle involved in the experiment, we will now give 

 a general description of the apparatus. In a common 

 glass flask, of carafe-shape, and flat-bottomed, of 500 c. c. 

 capacity (about 32 cubic inches) is poured 50 c. c. 

 (about 3 c. in.) of distilled water. The dimensions 

 are exclusive of the space occupijd by the air- 

 tight stoppper. Through this stopper are passed 

 two small tubes, at the end of one of which is attached 

 (a little to the side of the orifice) a small square silver 

 table of I square centimetre in are a (•16 sq. in.). The flask 

 is inverted and the small silver table is placed exactly 

 I c. (-39 in.) from the inverted bottom, so that the con- 

 tents of air right above the table is r c. c. ("064 c. in.). 



The observing table is divided by a very fine instrument 

 into 100 equal squares (i mm. a side). This silver plate 

 is very highly polished, but the burnishing is done all in 

 one direction, so that during the observations it may 

 appear dark when the fine rain- particles glisten with the 

 reflected light, in order that they may be easily counted. 

 The tube to which the silver mirror is attached, is con- 

 nected with two stopcocks, one of which can admit pure 

 filtered air from one vessel, and the other can admit 

 small measured portions (i c. c.) of the air to be ex- 

 amined. The other tube in the flask, whose extremity 

 does not reach so high as the first, is connected with an 

 air-pump of 150 c. c. caoacity (about 10 c. ins.). Over 

 the flask is placed a covering, coloured black in the inside. 

 In the top of this cover is inserted a powerful magnifying 

 glass, through which the particles on the silver table can 

 be easily counted. A little to the side of this magnifier, 

 is an orifice in the cover, through which light is concen- 

 trated on the silver mirror. This light, again, has had 

 to pass through a spherical globe of water, in order to 

 abstract the heat rays, which might injure the experi- 

 ment. 



This is a general description of the instrument. The 

 experiment is performed in this way : — The air in the 

 flask is exhausted by the air-pump. The flask is then 

 filled with air which has been thoroughly filtered (freed 

 from dust-particles). Into the flask is introduced a cubic 

 centimetre of the common air which is to be examined 

 for its dust. With one stroke of the air-pump what 

 takes place ? The air — mixture of 450 c. c. of filtered 

 air, and i c. c. of common air — is made to occupy an 

 additional space of 150 c. c. (the barrel of the air- 

 pump) ; and the diminished pressure allows the water- 

 vapour to rise from the surface of the water in the flask, 

 or from the walls which were moistened by the inversion 

 of the flask. The observer looking through the magni- 

 fying glass upon the mirror, sees the mist particles 

 (formed by the moisture attaching to the d 1st free-sur- 

 face) fall on the mirror. As these particles last for a time, 

 he can count the number on a single square in tv/o or 

 three places, and take the average on the squares. 

 Suppose the average number upon one of the squares 

 on the mirror were one, then on the plate there would 

 be 100 ; and the 100 particles of dust are those in 

 the cubic centimetre of mixed air. But as there are 

 600 c. c. of air, with which the i c. c. of the air to be 

 examined has been mixed, the number of dust-particles in 

 I c. c. of that air is 600 by 100, equal to 60,000. By 

 this method the number can be counted in any specimen 

 of air. 



From numerous experiments made by Mr. Aitken, he is 

 able to give the dust-particles in a thousandth part of 

 acubicinch of the air examined. In outside air the number 

 was 521, when the examination was made after a heavy 

 rainfall, and 2,119 when the air had been for some time 

 dry. This shows a very marked difference of four to one, 

 and this may yet be made available as a test of the 

 state of the atmosphere, as a help to meteorological in- 

 quiry. In his laboratory, when two lamps were burn- 

 ing, the number of dust-particles was 30,318 in the 

 thousandth part of a cubic inch, and when the air was 

 taken from near the ceiling, no less than 88,346 were 

 counted. But when the air rising from the flame of a 

 Bunsen burner was examined, he counted no less than 

 489,000 in the thousandth of a cubic inch. He is, more- 

 over, of opinion that these figures are under rather than 

 over stated. 



