58 
CONSIDERATION OF LIGHT. 
throiigli which it passes, unless that medium be susceptible of electro-chemica 
changes. 
This leads us to the consideration of that substitute for glass which wa‘ 
announced some time since, under tlie title of Whitney’s composition. It is nor 
perhaps fortunate, that Drake’s sheet-glass may soon be obtained at comparativelv 
small cost ; but still, the varnished canvas was a great acquisition, because tli» 
direct light of the full solar rfiys might thus be diffused throughout a house, or pit 
communicating its softened, genial temperature, without danger of scalding D 
lenticular action. But the cloth decays, cracks, perishes ; and not only so, i” 
becomes patchy, discoloured, and offensive to the eye. What are the causes of thesi 
disagreeable concomitants, which threaten to deprive the forcing gardener of one o 
his best appliances ? 
It will be found, that if a frame of varnished linen, or calico, be kept in | 
dwelling, unexposed to the evaporation of the soil underneatli, and to the actioi 
of moisture from above, the decay will rarely occur. Whereas when employed as { 
horticultural covering, it will speedily become blotched or stained all over witl^ 
fungus, lichen, or some such cryptogamous vegetation. Here then we perceiv( 
the agents of decay, and hence, if some ingenious person could incorporate with tlui 
varnish, or superpose u'pon it, when applied, a certain antidote, or repellant of tin 
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), botl 
inimical to mouldiness ; 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, mightj 
be so masked as to lose its protective qualities ; and the sublimate, if merely laic 
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 o| 
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 followingj 
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, 
