which gradually becomes deeper, and a dirt, inky-looking compound is ultimately produced. When this has taken place, chlorate of potash in fine powder must be dropped into the hot mixture very gradually until the black colour disappears. This must be done cautiously, for the action is so violent that much spurting is occasioned, and the liquid being very corrosive a tolerably capacious vessel should be used in order to keep the splashes within reasonable bounds, or serious damage to the operator's clothes may ensue.
The nearly decolourized liquid must now be diluted with a considerable quantity of water, and the deposit allowed to subside, the supernatant liquid poured off, and the process of heating with sulphuric acid and addition of chlorate of potash repeated, until the sulphuric acid occasions no further blackening, then the cleaning may be finished in the usual way by washing.
The chemistry of this process is thus explained:—When strong sulphuric acid is brought in contact with organic matter it abstracts the greater part of the water, setting free the carbon in an imperfectly dissolved, or, at least, finely divided state. Other changes also take place of a somewhat complex nature. The chlorate of potash being now added to this mixture is immediately decomposed by the sulphuric acid, supplying oxygen to the carbon, with which it forms carbonic acid gas, which escapes with some free chlorine. Other oxidizing agents may be used; nitrate of potash may be added in the same way as the chlorate, or nitric acid itself may be used, but they are all inferior to the chlorate. This process was published some few years back, and has been found by experience to be the most efficacious method of operating on guano for Diatomaceæ. The operator requires no small share of patience in working on guanos, for it sometimes happens that some samples yield none, or so few as to be scarcely worth the trouble of cleaning. When, however, the yield is good, the experimenter is well repaid for his time and materials.
In consequence of guano being so largely composed of organic matter it is well to work on a tolerably large quantity, say a pound, or, at all events, not less than four ounces. The writer has found the Peruvian and Californian to yield a very rich variety of these minute objects. It may be as well the mention that the above processes may be well applied to the cleaning of the siliceous cases of our native species. The matters containing them should be allowed to dry before being subjected to the action of the acids.
It is scarcely necessary to add a caution as to the corrosive nature of the acids here employed; the operations should also take place under a chimney or in a well-ventilated apartment. In case of accident apply plenty of cold water, then any alkali at hand, chalk, or whitening.
LEAF TEACHINGS.
By W. Wallace Fyfe.
Oersted observes, in his "Soul of Nature," that organic beings constitute the elementary, and inorganic the higher geometry of nature; and, although we meet with many regular mathematical forms in the leaves of plants, whether triangular, polygonal, cylindrical, spherical, or elliptical (for in what may be scientifically termed leaves we have all these), nevertheless, in the shapes of leaves and the flow of their venation we have seldom figures that are severely exact; yet, for all that, every atom of the most waving lines and outlines is capable of being reduced to a regular curve. It was Goethe who defined all the appendages of the plant as leaves[1]—transformed leaves. We do not go so far as the German poet and philosopher, but seem it sufficient to recognise the truth that all are modelled after the leaf. The form of the leaf, therefore, becomes a generic principle in structural botany. The structure of a leaf is founded on what have been termed architectural principles; but in truth it is architectural principles that have been derived from it, since Sir Joseph Paxton has acknowledged that to the ribbing so peculiarly adapted for bearing up the broad floating leaf of the vast Victoria Regia and its massive inflorescence, he was mainly indebted for the idea of his palace of glass, which has so wonderfully resisted, in its fairy-like frailty of appearance, the storms of time. If we are entitled, with Goethe, to extend this principle of structure from the leaf to the tree, we shall find that to it also Smeaton was indebted for the design of the Eddystone Lighthouse—a structure calculated to sustain weight and resist pressure—since he derived it from the bole of a tree. The elements of plant structure are found, however, in the cell—the lower forms of plants being simply cells separate and independent; the higher forms,
- ↑ Linnæus had previously affirmed the leaf to be the type of all the floral organs. Goethe's Versuch die Metamorphose der Pflanzen zu erklären was published in 1790.
