i86 



HA RD WICKE' S SCIENCE-GOSSIP. 



air to form fog -particles visible to the eye, when the 

 pressure is lowered so as to bring the temperature 

 down to the dew-point. If jets of steam be intro- 

 duced into two glass receivers, the one filled with 

 common air and the other filled with filtered air 

 (that is, air from which the dust-particles have been 

 removed by being driven through eotton-wool), the 

 first will become densely clouded, while the other 

 remains quite clear. It is want of dust-particles that 

 has prevented the formation of fog-particles in the 

 second, whereas they are very abundant in the other. 

 Dust, then, is necessary for the formation of fogs, 

 mist, or rain. 



The pocket instrument, when fitted up for use, 

 may be thus generally described : — A carefully 

 adjusted air-pump is so fitted up with a guide-collar, 

 that the drawing down of this collar to certain marks 

 enables the observer to introduce into the receiver 

 measured quantities of the air to be examined. The 

 glass-cylindrical receiver is a centimetre in height. 

 Fitted into the bottom is the glass counting stage, 

 and above it is a magnifying lens of about two-centi- 

 metre focus. The counting stage is divided into 

 squares of one millimetre a side. This is a delicate 

 process, and is effected by covering the thick glass 

 with a thin film of beeswax, and drawing the lines with 

 a fine needle-point, into which white hydrofluoric acid 

 is introduced. A circular disc is then cu from this 

 sheet of a suitable size for fixing into the bottom of 

 the receiver. This micrometer stage is illuminated 

 by the spot-mirror from below. This spot-mirror is 

 simply an ordinary lighting microscopic mirror with 

 a black circular space in the centre. This enables 

 the fog-particles that fall on the stage to be illuminated 

 by means of a slightly oblique light, while an image 

 of the black spot covers the field of the lens. The 

 result is that by the reflection of ordinary skylight 

 from the surface of the mirrored part the drops are 

 seen shining with brilliant opal hues on a nearly 

 black field, and are counted with great ease. To 

 make the fog-particles look like little round balls, 

 with intense internal reflection, instead of adhering 

 to the glass, and spreading themselves over it more 

 or less, the glass should have a little refined beeswax 

 put on it and rubbed off till its presence is scarcely 

 detected. The inside of the receiver is kept moist. 



When all is thus adjusted, the air in the receiver 

 must be purified in this way : — A stroke of the pump 

 is made ; this causes condensation to take place on 

 the dust-particles when some of them drop out of the 

 air. The piston is again put to its top position and 

 another stroke made, when more particles settle. 

 After this process has been continued a few times all 

 the particles of dust will have become nuclei, and be 

 deposited on the bottom of the receiver. The air in 

 the receiver will now be pure, no drops falling when 

 expansion is made. The necessary quantity of impure 

 air is then introduced into the receiver by sliding the 

 guide-collar of the pump. A little extra arrangement 



is here needed which cannot be minutely described. 

 There is a stirrer within for mixing the pure -and 

 impure air. On the pump being used, at one stroke 

 the expansion makes the dust-particles seize the 

 moisture in the air of the receiver, and fog-particles 

 drop on the stage to be counted by looking through 

 the magnifying glass. Suppose, for instance, that 

 the guide-collar were drawn down to the mark ^ on 

 the scale, and on the average of ten tests, two drops 

 were counted on each square millimetre. There 

 would be in that proportion 200 drops on the square 

 centimetre : and as the receiver is one centimetre in 

 height, there would be 200 dust-particles in the 

 centimetre. But this figure must in this case be 

 multiplied by 50 to get the number of particles in the 

 outer air, which in this case would be 10,000, that is, 

 156,250 dust-particles in the cubic inch. By this 

 method the number can be counted in any specimen 

 of air. The inumber can be counted in all places 

 from 34 in the cubic inch on the top of Ben Nevis to 

 4,000,000 in the cubic inch at Edinburgh. At Rigi 

 Kulm, near Lake Lucerne, they vary from 3360 to 

 37,600 in the same space. But they have been 

 counted up to 489,000,000 in a cubic inch of the air 

 arising from the flame of a Bunsen burner. Verily 

 the dust is numbered ! 



The dust-detector consists of a test-tube of metal 

 with glass ends about twenty inches long and one 

 inch in diameter, and an air-pump with half the 

 capacity of this tube. Next one end of the tube is a 

 passage by which it communicates with the pump, 

 and near the other end a stop-cock is fixed for 

 admitting the air for examination. The tube is lined 

 with moist blotting-paper. By a stroke of the air- 

 pump the air is rarified, and the dust-particles seize 

 the moisture in the air and from the inner surface 

 forming fog-particles : looking down the tube, the 

 observer is astonished to find a blue colour in the 

 fog. 



The dust-counter is employed to graduate the dust- 

 detector. The eye is able to detect many different 

 shades of blue, when it cannot assign names to the 

 hues. With few dust-particles in the air to be tested, 

 the colour seen in the fog of the detector is pale blue, 

 and as the particles increase in number the blue in- 

 creases in depth. On the outside of the dust-detector 

 the different shades of blue are fixed in order, and 

 opposite the hues are marked the number of dust- 

 particles as found by the dust-counter when testing 

 the air which produced the several hues. When the 

 number of dust-particles in the cubic inch is ^ of a 

 million, the detector indicates " blue just visible : " 

 when i£ millions, "very pale blue;" when S 

 millions, "pale blue;" when 24 millions, "'fine 

 blue;" when 40 millions, "deep blue;" and when 

 64 millions, "very deep blue." When making a 

 sanitary inspection, any increase in the depth of blue- 

 above the normal would indicate the amount of the 

 increase of pollution of the air ; and the figures on 



