No. 9. 



Consideration of Light. 



269 



neutralize each otlier. In conformity witli 

 these chemical phenomena, light may be 

 considered as exerting an invisible attrac- 

 tion upon the points and foliage of vegeta- 

 bles, which induces tlie flow of the sap in 

 an ttpwdrd direction, and also the lateral 

 courses of the fluids through the tissue of 

 the cells, and that of the medullary process- 

 es, thus producing the laboratiou and con- 

 coction of the proper specific juices of each 

 individual plant. 



There must be a governing agent, — and 

 therefore we repudiate that maudlin lan- 

 guage which ascribes volition and choice to 

 inanimate structures. 



2. Temperature, its rise and fall, are dis- 

 cerned as measured by our very imperfect 

 instruments. In nine cases of ten, temper- 

 ature is only a result of e.xcited chemical 

 action, of which combustion is one form ; 

 but as referred to the sun, the manifestation 

 of direct heat is generally ascribed to the 

 red rays, while those of magnetism are 

 found in the blue and violet. Siill the en- 

 tire solar, or white ray, diffuses heat and 

 light, without communicating heat to the 

 transparent medium through which it pass- 

 es, 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 Whit- 

 ney's composition. It is now perhaps fortu- 

 nate, that Drake's sheet-glass may soon be 

 obtained at comparatively small cost; but 

 still, the varnished canvass was a great ac- 

 quisition, 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 scald- 

 ing by lenticular action. But the cloth de- 

 cays, 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 var- 

 nished linen, or calico, be kept in a dwell- 

 ing, unexposed to the evaporation of the 

 soil underneath, and to the action of moist- 

 ure 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 incor- 

 porate with the varnish, or superpose upon 

 it, when applied, a certain antidote, or re- 

 pellant 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 subli- 

 mate,) both 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, 

 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. Per- 

 haps a thin varnish of caoutchouc (India 

 rubber) dissolved in pale naphtha, would be 

 found to act efficiently 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 not heat the linen 

 screen: hence, chemical action is induced, 

 and strange to say, under these circum- 

 stances cryptogamous vegetation preys upon 

 the tissue: we have much to learn in cir- 

 cumstances of every-day occurrence. 



The late president, Mr. Knight, was ex- 

 ceedingly curious in his endeavonrs to dis- 

 cover the causes of particular phenomena. 

 He thought that gravitation had a very im- 

 portant influence on the growth of plants; 

 and Professor Davy, with whom he was in- 

 timate, recounts one of Mr. Knight's expe- 

 riments, 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 gar- 

 den-bean on the circumference of a wheel, 

 which in one instance was placed vertically, 

 and in the other horizontally, and made to 

 revolve by means of another wheel worked 

 by water, in such a manner, that the number 

 of the revolutions could be regulated ; the 

 beans were supplied with moisture, and 

 were placed under circumstances favourable 

 to germination. The beans all grew, not- 

 withstanding the violence of the revolution, 

 which was sometimes as much as 250 revo- 

 lutions a minute on the vertical wheel — 

 which always revolved rapidly — and with 

 little variation of velocity; the radicles, or 

 roots, pointed precisely in the direction of 

 radii in whatever direction they were placed. 

 The germs took precisely the opposite direc- 

 tion, and pointed to the centre of the wheel, 

 where they soon met each other. Upon the 

 horizontal wheel the conflicting operation of 

 gravitation and centrifugal force, occasioned 

 the germs to form a cone more or less ob- 

 tuse, according to the velocity of the wheel, 

 the radicles always taking a course diamet- 

 rically opposite to that taken by the germs, 



