those of rusting and additional expense, cannot be denied. The reply is, the smaller the
bars, the less their pov'cr of conducting ; and a thick coat of paint, and the covering of
half the bar by the putty requisite to retain tho glass, also lessens this power: it is
added, heat may be supplied by art; but solar light, the grand advantage gained by
metallic bars, cannot, by any human means, be supplied otherwise than by the transparency
of the roof.
A ttra c tio n o f electricity. To this objection it is replied, that if metallic hothouses
attract electricity, they also conduct it to the ground, so that it cannot do any harm.
Also, that no instance can bo produced of iron hothouses having been injured by the
effects of tliis fluid.
Su b se ct. 4. G la zin g o f Hothouse Boofs.
2089. T h e glazing o f plant-houses has ahvays been a subject of great interest and
of considerable difficulty to gardeners. When tender exotic plants were first groivn
in this countiy, the houses in which they were sheltered having only glass fronts, the
sashes composing them only differed fi-om those of ordinary windows in being somewhat
larger; but -«'11611 glass roofs were introduced, numerous difficulties presented themselves,
whicli rendered the ordinaiy mode of glazing windows impracticable for the
sashes of plant-houses. The sashes forming the roofs of tlicse houses being at an angle
of not more than 45°, and generally less, the glass in them was liable to be broken by any
heavy substance that might chance to fall upon the roof; and in -violent hailstorms, particularly,
it was a conunon occurrence for almost every pane of glass in the roofs of the
plant-houses exposed to them to be broken. The most obvious way of remedying this
evil was, to make the glass used for the roofs of plant-houses exceedingly thick ; but
for many years this could not be done without incurring an enormous expense, as the
duty on glass was paid by its weight, while it was sold to the consumer by measure, and
consequently it became, as Dr. Lindley observes, “ the interest of the manufacturer to
blow his glass to the thinnest possible state, and thus to increase, to the greatest degree, the
brittleness of the most fragile commodity we know of.” {G a rd . C hron. for 1845, p. 115.)
The consequence of these difficulties was to induce gardeners to use thcir glass in very
small panes; and this plan, though it entailed on cultivators many veiy serious inconveniences,
was generally followed till the year 1845, when che duty on glass ivas taken
off', and a great impulse was given to the glass trade by the extraordinary quantities that
were required for horticultural pui-poses. Among the various inconveniences wliich had
been occasioned by the glazing with small panes, that of obstructing the light was always
considered one of the most serious. Iu glazing with small panes, it was necessary to
make one lap over tlie other, to exclude the cold; and these laps were inconvenient in a
.great variety of ways. Water was continually lodging between them, and generating
masses of minute confervce and other similai- plants, -which obstructed the light; and in
irosty weather, the moistm-e between the laps was liable to freeze, and to break tho glass.
The great difficulty, however, was the obstruction of light; for the necessity of light to
plants is too obvious to every cultivator to admit of any dispute. In consequence of this,
-when the duty -was taken off glass, the first idea seems to have been to have the panes
of glass nsed for horticultural purposes as large and as thick as possible; and, accordingly,
we find that houses were glazed with immense panes, 2 or 3 feet long, 12 inches
wide, and weighing fi-om 18 to 26 ounces the square foot. Of course the light in
houses glazed in this manner was almost as brilliant as in the open air; and it was
confidently expected that this gi-eat improvement in glazing would foi-m quite a new
era in the cultivation of exotic plants. Time, however, which destroys so many
brilliant liypothescs, has proved the fallacy of this; and it is found that the new mode
of glazing is attended with difficulties which are quite as serious as those which pro-v-ed
so annoying to the cultivator of the old school. The principal of these is, the scorching
ofthe plants; and minor ones are, the twisting and consequent Ql-working of the frames
of the sashes from the great weight and length of the glass.
2090. B r itis h sheet glass was one of the first kinds generally employed in hothouses
after the reduction of the duties. This glass being cast in plates of any size or thickness
required, appeared exactly suited to meet the wishes of cultivators who required large
panes; and it seemed admirably suited for horticultiiral purposes, jis this kind of glass
was beautifully clear and quite free from any colour. It was soon found, however, that
in hothouses where this glass had been employed, the more delicate kinds of plants were
“ injm-cd by a sort of scorching when under the influence of the sun’s rnys in pai'ticular
aspects.” Various causes were assigned for this effect; and, among otliers, Mr. Hunt,
Keeper of the Mining Records at tho Museum of Economic Geology, wlio had tried a
great many experiments on tlie effects produced by coloured transparent media on the
growth of plants, suggested the employment of tinted glass. To try the efficacy of his
suggestion, Mr. Iliint had a number of small panes of glass manufactured of different
colours, and endeavoured, by various expei-iments, to ascertain the effects of the different
colours on the growth of plants.
2091. According to M r . H u n t’s hypothesis, the solar rays produce three distinct classes
of phenomena, viz. light, heat, and chemical agency or actinism; and these phenomena,
lie thinks, are produced by different coloured rays. Thus, he considers that the yellow
are the luminous rays, that the red rays produce heat, and that chemical action, or
actinism, is produced by the blue rays; and, foUowing up this idea, he thus describes the
effects which he supposes the different coloured rays produce upon plants : — “ Yellow
lig h t, or luminosity, impedes germination, and accelerates that decomposition of carbonic
acid wliich produces wood and woody tissues. Under its influence leaves are small,
and wood short-jointed. B e d lig h t (heat) carries heat, is favourable to germination, if
abundance of water be present increases evaporation (or perspiration), supports the
flowering quality, and improves fruit. Under its influence colour is diminished, and
leaves ai-e scorched. B lu e lig h t (chemical action or actinism) accelerates gei-mination
and causes rapid growth. Under its influence plants become weak and long-jointed.”
2092. In consequence o f the experiments o f M r . H u n t, a vei-y clear pale yellowish gi-ecn
1 ,1 . . 1 ....... _ £■ 1_- ^ ...., the colour of which was given by __o_xizdvleor aoivfir ..c o*p■'per, was selected for glazing the
great palm-house at Kew; that house being so large that it was almost impossible to
adopt any system of blinds for shading the plants that was likely to be efficacious. The
green glass chosen was so transparent, that scarcely any light was intercepted. On examining
the spectral rays through it, it was found that the yellow was slightly diminished
in intensity, and that the lower edge of the ordinai-y red ray was cut off. It also appeared
to have a remarkable action upon the non-luminous heat rays. “ The absence of the
oxide of manganese, commonly employed in all sheet-glass, was insisted upon, it having
been found that glass into the composition of which manganese enters will, after exposure
for some time to intense sun-light, assume a pinky line; and it was found that the
slightest approach to redness would allow the passage of those heat rays which were
found to have so remarkably scorching and injurious an effect.
2093. Coloured glass, however, notwithstanding the ingenious hypothesis which has
been broached respecting it, does not appear to have succeeded in practice, and, indeed,
none of the new kinds of glass that have been used since the reduction of the duty
appeal- to have answered the expectations formed of them. Indeed, as Dr. Lindley
very justly observes, “it is improbable, to say the least of it, that any ai-tificial light
should be as well suited to plants as that which has been provided for them by the great
Author of the universe.” “Plants in hothouses arc, however, neccssai-ily exposed to a
light rendered iu some degree artificial by the glass through which it passes ; and therefore
it is really a serious question, what kind of glass is best suited to the pm-pose.’"
Dr. Lindley adds, that his own opinion is “in favour of the whitest, as being that in
which the three primitive colours arc most entirely blended.” And this opinion will
probably, in the end, prove to be con-ect, as light passing tb-oiigh a perfectly colom-less
medium must be most like the pure light of the open air.
2094. T h e scorching o fth e leaves o f p la n ts under the large panes o f glass. Mi-. Hunt,
as we have already seen, attributes tliis result to the transmission of the red or hcat-
gi-ring i-ays, and thinks that if their progress could be impeded, the cause would be removed.
Dr. Lindley, in the same vei-y able ai-ticle in the Gardeners’ C hronicle from which
we have already quoted, suggests that “the scorching which has been complained of has
arisen from a high temperature caused by imperfect ventilation. In the old cro-wn-glass
houses the panes were smidl, the laps large and nnmcrous; and thus an abundance of
apertures existed through which heated air could fi-cely pass. In sheet-glass houses the
panes arc long, the laps few or none, and the means of escape for heated air reduced to
a minimum. In many cases, no sufficient provision is made for the removal of the di-y
and scorching heated air. Hence arise unexpected accidents, which have been en-one-
onsly ascribed to the quality of the glass rather than to tho imperfect construction of
the houses.” (Gard C7won. for 1848, p. 155.)
2095. T he uneven surface o f sheet-glass has been supposed the principal cause of the
scorching of the leaves of plants grown nndcr it. In June, 1848, Mi-. Mitchell, gardencr
to H. Willyams, Esq., of Carnanton, finding that his plants were scorched veiy ii-regularly,
examined the glass of his houses, and found that in some places the glass was mucli
tliicker than in others, so that these parts of the glass acted as a burning lens upon the
leaves of tho plants. “This effect,” Dr. Lindley observes, “arises from the peculiar
mode in which sheet-glass is manufactured. When sheet-glass is made, the first operation
is to form a cylinder, which is afterwards slit on one side, and spread into a flat
sheet. Now, if we assume a cylinder, of 26 oz. glass, to have its sides 4 of an inch thick,
the inner circumference will be | of an inch less than the outer. But when the cylinder
is spread into aflat sheet, the two surfaces become of an ecpial -\vidtli, the glass_adjusting
itself by the expansion of the inner or smaller surface, ancl by the contraction ot the
outer or larger surface. In this operation are formed what the manufactui-crs call