188 POUCHET, ON ATMOSPHERIC MICROGRAPHY, 



spicula in my possession, and in these cases we cannot 

 reasonably attribute them to the action of parasitic forms 

 of vegetation. 



When these minute tubuli are completely separated from 

 their matrix, they still preserve all the characteristic appear- 

 ance of ceratode, and their thickened glue-like aspect is 

 very dissimilar to the delicate translucent forms of minute 

 vegetable tissues. 



The young and immature fibre of the true sponge, much 

 less in diameter than the adult fibre, are usually destitute of 

 the network of tubular structure that surrounds the others; but 

 this we can scarcely imagine would be the case, if it were 

 due to a parasitic vegetation. The adult fibres are also often 

 partially destitute of the tubular fibre, in consequence of 

 its having been stripped of the sheath that surrounds it, on 

 the inner surface of which these minute tubular fibres are 

 embedded. The stripped fibres of the skeleton may easily 

 be recognised by the faint diagonal lines upon them, which 

 are not visible through the external coat of the vegetable 

 fibre. 



Similar tissues to those in sponges which I gave to 

 the author occur also in siliceous fossil sponges. These 



1 have figured and described in the ' Annals and Magazine 

 of Natural History,' vol. x, pp. 18 and 84, Plate III, figs. 



2 and 6, from green jaspars from India, and 3 and 4 

 from chalk flints. An identity of organization may naturally 

 be expected to occur between the recent and fossil sponges ; 

 but it is difficult to conceive these tubes to be effected 

 by parasites under such widely different circumstances of 

 time and place as those of the fossil and recent specimens. 



Atmospheric Micrography. On the Means by which all the 

 Corpuscles normally invisible, contained in a Deter- 

 minate Volume of Air, may he collected into an infinitely 

 small Space. By M. Pouchet. 



('Comptes rendus,' April, 1860, p. 748.) 



I have succeeded, by means of a very simple instrument, 

 in concentrating, upon an infinitely minute surface, all the 

 solid and normally invisible corpuscles floating in the atmos- 

 phere, so as to allow of their being strictly appreciated and 

 counted. By this means we can, if we please, concentrate on 



