Correspondence. 113 
It would not be possible to mention here the important facts of this 
memoir; the results below are from a note which M. Chevreul has an- 
nexed to his paper. e whole will appear in the Memoirs of the 
Academy of Science. 
This note relates to the examination of the black material found 
under the pavements of the streets of Paris and between the stones. 
te) 
It owes its origin to the iron rubbed off from the tires of carriage 
wheels and shoes of horses, and is carried into the pavement by the 
rains. In its fine state of division, the iron readily oxydizes under the 
agency al 
then peroxyd; and moreover, from the analysis of this material, it has 
been found that there is a large quantity of sulphuret of iren. 
presence of such a bed in the soil, tends, according to M. Chevreul, to 
deprive of oxygen the air which penetrates it, and it should therefore 
be considered an obstacle to the salutary influence of the atmospheric 
with plaster in a flask not hermetically sealed, at the end of seven years 
is changed into black magnetie oxyd with a little ammonia. If albu- 
ars 5 
Some seconds to the light, the original design of iodid of starch is changed 
into iodid of silver; and by further exposure to the light, this iodid, be- 
ing much more sensitive than the nitrate of silver contained in the paper 
or in the layer of starch on the glass, is acted upon before the nitrate 
it is then only needed to plunge the glass or paper into a solution of 
gallic acid to bring out the original design, which is then treated with 
hyposulphite of soda, just as for photographic pictures. The pictures 
are th red as permanent as ordinary photographs. 
Sxconp Senses, Vol. XVI, No. 46.—July, 1853. 15 
