POUCHET, ON ATMOSPHERIC MICROGRAPHY. 131 



ments of the tissue of various plants ; a few ligneous fibres ; 

 more frequently fragments of cells and vessels; often hairs of 

 the nettle and other plants ; numerous filaments of cotton, 

 usually white, but sometimes of various colours, also derived 

 from articles of dress ; some fragments of anthers and pollen- 

 grains of malvaceous plants, of Epihbium and Pinus ; spores 

 of cryptogamous plants, but in very small number. Lastly, 

 I have constantly noticed, and almost invariably where my 

 observations have been extensive, a very notable quantity of 

 wheat-starch mixed with the dust, whether recent or old; 

 and, in rare instances, may be found the starch of oats, 

 barley, and the potato. 



It is evident, therefore, that the atmosphere holds in sus- 

 pension a certain quantity of ivheat-starch among its dust- 

 corpuscles. This substance is met with in all places where 

 it enters into articles of food, and it may readily be distin- 

 guished by its physical and chemical characters. The grains 

 of which it consists are sometimes ovoid, sometimes spherical ; 

 in diameter they usually vary from 0*0140 to - 0280mm. 

 Besides these numerous extremely minute incipient granules, 

 may be seen others less than - 0028mm. in diameter. The 

 larger grains are very rare; those of a medium size far 

 more common, and the very minute ones extremely abundant. 

 In the large granules the concentric layers and hilum may 

 sometimes be readily distinguished. It is rather curious to 

 remark that this starch, notwithstanding, in some instances, 

 its secular existence, still affords all the physical and chemical 

 properties of the recent substance. The only difference 

 being that the very ancient presents a light-yellow tint. 

 When boiled in water it swells and dissolves. Very weak 

 hydrochloric acid has no effect upon it ; it is coloured blue 

 by iodine with greater or less intensity ; and sometimes its 

 colour disappears under the influence of light. One circum- 

 stance which has struck me, is, that among starch found in 

 dust several centuries old, I have, from time to time, met 

 with grains which had spontaneously assigned a beautiful clear- 

 violet colour. Was this due to the influence of time, or to 

 the vicinity of the sea, or, lastly, according to M. Chatin, to 

 the traces of the vapour of iodine contained in the atmo- 

 sphere? Finally, that no doubt may be entertained with 

 respect to the identity of this aerial fecula with ordinary 

 starch, I would add, that its effect upon polarized light is 

 the same, except that, when procured from a very ancient 

 deposit, its polarizing property is less energetic. 



It is evident that it is this fecula thus perfectly charac- 

 terised by its physical and chemical properties, that M. De 



