195 
I need not add that the most careful centering and adjust- 
ment are essential. 
In preparing the object I made a section of a small por- 
tion of the leaf laid on a piece of cork in water under a 
simple microscope, separating only one layer of cells ; I laid 
this on a slip made of thin covering glass, and over it 
applied a thin glass cover, a small feeding bottle and thread 
being made use of to prevent the object being dried by 
evaporation. 
I then covered the stage and object with a piece of black 
velvet, to prevent interference from other lights in the 
room. 
Mr. Sidebotham reported the plentiful occurrence, in 
J une, of Thlaspi arvense, L., on the borders of Carrington 
Moss. It has very rarely been noticed in the neighbour- 
hood of Manchester. 
ON THE ABSORPTION OF GASES BY CHARCOAL. 
The following Letter from Dr. R Angus Smith, F.RS., 
to Dr. J. P. Joule, F.RS., was received by the Editor, June 
20th, 1867. 
Manchester, June 17th, 1867. 
My Dear Joule, 
You asked me about my experiments on the absorp- 
tion of gases by charcoal. I certainly seem to delay them, 
but I have little spare time. In 1848 I illustrated the 
oxidising power of porous bodies, referring chiefly to sand. 
In 1862, when speaking of the absorption of gases by char- 
coal, I ventured to say that the physical and chemical action 
could not be separated. I have been anxious to obtain 
more direct evidence. 
