362 ANNUAL OF SCIENTIFIC DISCOVERY. 



THE COMPOSITION OF ATMOSPHERIC DUST. 



The composition of atmospheric dust will always be of two kinds, inor- 

 ganic and organic, that is to say, mineral particles, and the skeletons of 

 animalcules, or the skeletons and seeds of plants. The mineral particles will 

 of course depend on the nature of the soil and position of the spot whence 

 the dust was derived. It may be swept in from the gravel walks of a gar- 

 den, from the highroad, or from the busy street. The grinding of vehicles, 

 the wear of busy feet, the disintegration everywhere going on, keep up a 

 constant supply of dust. The smoke of chimney and factory, steamship 

 and railway, blackens the air with coal-dust. If silicious rocks are not a 

 great way off, we shall find abundance of particles of silica, with sharp 

 angles, sometimes transparent, sometimes yellow, and sometimes black. 

 And this silica will occasionally be in so tine a powdered condition, that the 

 granules will look like very minute eggs, for which, indeed, many microscop- 

 ists have mistaken them. In this doubt we have recourse to chemistry, and 

 its tests assure us that we have silica, not eggs, before us. Besides the 

 silica, we see chalk in great abundance; and, if near a foundi\v, we shall 

 certainly detect the grains of oxide of iron (rust), and not a little coal-dust. 

 Our houses, our public buildings, and our pavements, are silently being 

 worn away by the wind and weather, and the particles that are thus torn 

 off are carried into the dnst-clouds of the air, to settle where the wind list- 

 eth and the housemaid neglecteth. 



There is one thing which will perhaps be found in every place, and in 

 every pinch of dust, which is not a little surprising. It is starch. No 

 object is more familiar ro the microscopist than the grain of starch. It is 

 sometimes oval, sometimes spherical, and varies in size. The addition of a 

 little iodine gives it a blue color, which disappears under the influence of 

 light. There seems to be no difference between the starch grains found in 

 the dust of Egyptian tombs and Roman temples, and that found in the 

 breakfast-parlor of to-day. They both respond to chemical and physical 

 tests in the same way. But there is one curious fact which has been 

 observed by M. Pouchct of Rouen, namely, that in examining the dust of 

 many centuries he has sometimes found the starch grains of a clear blue 

 color; and he asks whether this may not be due to the action of iodine in 

 the air, traces of which, M. Chatin says, always exist in the air. The objec- 

 tion to this explanation is, that if iodine is always present in sufficient quan- 

 tities to color starch, the grains of starch should often be colored, whereas 

 no one but M. Pouchet has observed colored grains, and he but rarely. M. 

 Pouchet tells us that, amazed at the abundance of starch grains which he 

 found in dust, he set about examining the dust of all ages, and all kinds of 

 localities, the monuments and buildings of great cities, the tombs of Egyp- 

 tian monarchs, the palaces of the age of Pharaoh; nay, he even examined, 

 some dust which had penetrated the sculls of embalmed animals. In all 

 these places starch was found. But a moment's reflection dispels the mar- 

 vellousness of this fact. Starch must necessarily abound, because the 

 wheat, barley, rice, potatoes, etc., which form everywhere the staple of 

 man's food, are abundant in starch; the grains are rubbed off, and scattered 

 by the winds in all directions. 



So widely are these grains distributed, that a careful examination of our 

 clothes always detects them. Nay, they are constantly found in our hand-*, 

 though unsuspected until their presence on the glass slide under the micro- 



