GARDENING AS A SCIENCE. 
Our correspondent tells us that he constantly has grown great numbers of the 
Hydrangea in Italy, with both pink and blue flowers, as they are particularly well 
adapted to ornament the internal courts of houses where the sun never reaches. 
We stop here to notice the effect of shade, for we recollect to have once been 
told by a nurseryman, that he had known blue heads of flowers to have been pro- 
duced with pink heads upon the same plants, and at the same time, by entirely 
excluding the sun from the former. We attach little faith to the assertion ; but it 
is unquestionably true that shading is an indispensable condition, whenever it is an 
object to preserve the bloom in perfection. 
After having tried different agents to induce the blue colour, we are told that 
" pure iron filings, mixed with the compost in which the plants grew, were found 
most effectual ; but considering that the finer the iron the greater in all probability 
would be its activity, from the more intimate admixture of the particles with the 
various constituents of the compost, the fine sandy substance that is found under 
the stone wheels of the knife-grinders was substituted for common iron-filings, 
and with such success, that plants treated with the grinder's dust, gave blue 
flowers for three successive years without requiring any additional supply, though 
the intensity of the blue tint was greatest during the first two years." 
Such was the substance of the first communication; and it appeared so striking, 
that we became desirous to ascertain, as correctly as possible, the nature of the 
compost with which iron-filings in the first instance, and grinders dust, sub- 
sequently, had been blended. 
We were speedily favoured with a reply to our application, the substance of 
which is now given almost verbatim in the writer's own words : — 
44 The nature of the compost employed by me cannot be given with very great 
exactness, as Italian gardeners are not nice enough to mix their ingredients by 
measurement ; but I think that I am not very far from the truth, when I thus 
describe the compost. 
44 Common mould, such as would be used for geraniums, four parts ; rotten 
chesnut-wood, three parts ; and grinder's dust, four parts. A good substitute for 
rotten chesnut-wood, might be fibrous peat, such as is used for Orchidacea?. 
44 How the iron acts in the economy of the Hydrangea, man may never be able 
to discover, as Nature veils her laboratory too closely from human eyes ! Bog- 
earth might influence the colour of Hydrangea hortensis, through the iron, or 
perhaps the tannin, of which it contains so much." 
By the term bog-earth, our correspondent evidently intends to express the 
peat of turbaries, a substance not at all resembling the heath-soil so erroneously 
called peat by our modern writers. Newbury-peat was analysed by Davy ; and 
other substances from the peat-mosses have subsequently been investigated, and 
their constituents accurately detailed ; but few persons will meet with such, and, 
therefore, we must confine our remaining observations to the operation of the iron 
referred to in the foregoing quotations. 
