352 SCIENCE IN SffORT CHAPTERS. 



layender plant, or a hardy heliotrope to each of its mem- 

 bers ; so that a couple of million of such ozone generators 

 should breathe their sweetness into the dank and dead at- 

 mosphere of the denser central regions of London. 



A little practical experience of the difficulty of growing a 

 clean cabbage, or maintaining alive any sort of shrub in 

 the midst of our soot-drizzle, satisfied me that the mission 

 would fail, even though the sweetbriars were given aAvay by 

 the district visitors ; for these simple hardy plants perish in 

 a mid-London atmosphere unless their leaves are periodi- 

 cally sponged and syringed, to wash away the soot particles 

 that otherwise close their stomata and suffocate the plant. 



It is this deposit that stunts or destroys all our London 

 vegetation, with the exception of those trees which, like the 

 planes have a deciduous bark and cuticle. 



Some simple and inexpensive means of protecting vege- 

 tation^from London soot are, therefore, most desirable. 



When the Midland Institute commenced its existence 

 in temporary buildings in Cannon Street, Birmingham, in 

 1854, I was compelled to ventilate my class-rooms by tem- 

 porary devices, one of which was to throw open the existing 

 windows, and protect the students from the heavy blast of 

 entering air by straining it through a strong gauze-like fabric 

 stretched over the opening. 



After a short time the tammy became useless for its in- 

 tended purpose ; its interstices were choked with a deposit 

 of carbon. On examining this, I found that the black de- 

 posit was all on the outside, showing that a filtration of 

 the air had occurred. Even when the tammy was replaced 

 by perforated zinc, puttied into the window frames in the 

 place of glass panes, it was found necessary to frequently 

 wash the zinc, in order to keep the perforations open. 



The recollection of this experience suggested that if a 

 gauze-like fabric, cheaper and stronger than the tammy, can 

 be obtained, and a sort of greenhouse made with this in the 

 place of glass, the problem of converting London back- 

 yards into gardens might be solved. 



After some inquiries and failures in the trial of various 

 cheap fabrics, I found one that is already to be had, and 

 well adapted to the purpose. It is called "wall canvas," 



