CONSIDERATION OF LIGHT, 
through which it passes, unless that medium be susceptible of electro-chemical 
changes. 
This leads us to the consideration of that substitute for glass which was 
announced some time since, under the title of Whitney's composition. It is now 
perhaps fortunate, that Drake's sheet-glass may soon be obtained at comparatively 
small cost ; but still, the varnished canvas was a great acquisition, because the 
direct light of the full solar rays might thus be diffused throughout a house, or pit, 
communicating its softened, genial temperature, without danger of scalding by 
lenticular action. Bu?) the cloth decays, cracks, perishes ; and not only so, it 
becomes patchy, discoloured, and offensive to the eye. "What are the causes of these 
disagreeable concomitants, which threaten to deprive the forcing gardener of one of 
his best appliances ? 
It will be found, that if a frame of varnished linen, or calico, be kept in a 
dwelling, unexposed to the evaporation of the soil underneath, and to the action 
of moisture from above, the decay will rarely occur. Whereas when employed as a 
horticultural covering, it will speedily become blotched or stained all over with 
fungus, lichen, or some such cryptogamous vegetation. Here then we perceive 
the agents of decay, and hence, if some ingenious person could incorporate with the 
varnish, or superpose upon it, when applied, a certain antidote, or repellant of the 
invader, the varnished screen might last for years, sound and in a decent condition. 
We have thought of creasote, and bichloride of mercury (corrosive sublimate), both 
inimical to mould in ess ; but it does not as yet appear how either of them could be 
satisfactorily applied, because the former, if blended with the bulk of varnish, might 
be so masked as to lose its protective qualities ; and the sublimate, if merely laid 
over the varnish as a wash, would be carried away by rain from the upper surface, 
and by watery vapour from the under surface. Perhaps a thin varnish of 
caoutchouc (Indian rubber) dissolved in pale naphtha would be found to act effi- 
ciently in every way. 
We have said that the sun does not heat the glass through which it passes—- 
and glass is an electric ; but it does heat the linen screen : hence, chemical action 
is induced, and, strange to say, under these circumstances cryptogamous vegetation 
preys upon the tissue : we have much to learn in circumstances of every-day 
occurrence. 
The late President, Mr. Knight, was exceedingly curious in his endeavours to 
discover the causes of particular phenomena. He thought that gravitation had a 
very important influence on the growth of plants ; and Professor Davy, with 
whom he was intimate, recounts one of Mr. Knight's experiments which led him to 
presume that they owe the peculiar direction of their roots and branches entirely 
to this force. We refer to Davy's Second Agricultural Lecture for the following 
particulars : — 
Mr. Knight " fixed some seeds of the garden-bean on the circumference of a 
wheel, which in one instance was placed vertically, and in the other horizontally, 
