DUST AND FRESH AIR. 239 



is less, the mercury sinks in the tube, and the barometer is said 

 to fall. Therefore, every change of height of the quicksilver 

 which we observe is a sign and measure of a change in the vol- 

 ume of air around us. Further, this change in volume tells no 

 less upon the air inside our cases and cupboards. When the ba- 

 rometer falls, the air around expands into a larger volume, and 

 the air inside the cupboard also expands and forces itself out at 

 every minute crevice. When the barometer rises again, the air 

 inside the cupboard, as well as outside, condenses and shrinks, 

 and air is forced back into the cupboard to equalize the pressure ; 

 and, along with the air, in goes the dust. The smaller the crevice, 

 the stronger the jet of air, the farther goes the dirt. Witness 

 the dirt-tracks so often seen in imperfectly framed engravings or 

 photographs. Remember, ladies and gentlemen, whenever you 

 see the barometer rising, that an additional charge of dust is 

 entering your cupboards and drawers. So much for the barom- 

 eter, which is a very restless creature, rarely stationary for many 

 hours together. But this is not all. We also have the ther- 

 mometer. The temperature of our rooms varies daily often con- 

 siderably between midday and midnight, and greatly between 

 summer and winter. What does the thermometer tell us ? Not 

 less than the barometer does it tell of change of volume of the 

 air, though it is probably not so rapid in its effect upon the air 

 in inclosed spaces as is the change of volume indicated by the 

 barometer. Many of you have seen a fire-balloon. The heated 

 air, filling the balloon, expands, and becomes lighter than the sur- 

 rounding air, and up goes the balloon, until, the source of heat 

 having become exhausted, the contained air cools, contracts, be- 

 comes as heavy as the surrounding air, and down comes the 

 balloon again. So, also, as temperature rises outside our cases, 

 the increased warmth is slowly conducted to the air inside the 

 case, which expands and escapes through the crevices. Then, 

 when the time for cooling comes, the air inside slowly contracts, 

 and back rushes the air through the crevices, and again in goes 

 the dust. Thus, we see we have two factors constantly acting, 

 one or other tending to produce daily, nay, hourly, changes in 

 volume of our dirt-carrying air. 



In order to inform myself of the amount of change of volume 

 that could, under extreme conditions, possibly take place, I asked 

 Prof. Riicker to kindly calculate for me the change of volume 

 that would take place in one hundred cubic feet of air, between a 

 temperature of 30, i. e., just above freezing-point, in combination 

 with the barometer standing at thirty inches, or about " fair," and 

 a temperature of 60, combined with the barometer standing at 

 twenty -nine inches, or " stormy." He told me that the difference 

 would be about ten cubic feet, or one tenth ; in other words, that 



