These vessels, which are in nature cisterns of sap for the feeding the growth of the whole tree, are so large, that they are capable of being filled with coloured wax, in the manner of the vessels in anatomical injections; and this way they present pleasing objects for the microscope, and afford excellent opportunities of tracing their course and structure.
A METHOD OF FILLING THE SAP VESSELS OF PLANTS.
A great many shoots of the scarlet and other oaks are to be taken off in the Spring; they must be cut into pieces of about two inches in length, and immediately from the cutting they must drop into some warm rain water: in this they are to stand twenty-four hours, and then be boiled a little. When taken out, they are to be tied on strings, and hung up in a place where the air passes freely, but the sun does not shine. When they are perfectly dry, a large quantity of green wax, such as is used for the seals of law deeds, is to be gently melted in an earthen pipkin set in water; the water to be heated and kept boiling. As soon as the wax runs, the sticks are to be put in, and they are frequently to be stirred about. They must be kept in this state about an hour, and then the pipkin is to be taken out of the water, and set upon a naked fire, where it is to be kept with the wax boiling for two or three hours; fresh supplies of the same wax being added from time to time.
After this it is to be removed from the fire, and the sticks immediately taken out with a pair of nippers; when they are cold, the rough wax about them is to be broken off. Both ends of each stick are to be cut off half an inch long, and thrown away, and the middle pieces saved. These are then to be cut in smaller lengths, smoothed at the ends with a fine chissel, and many of them split in various thicknesses.
Thus are obtained preparations, not only of great use, but of wonderful beauty. Many trees this way afford handsome objects as well as the oak; and in some, where the sap vessels are few, large, and distinct, the split pieces resemble striped satins, in a way scarce to be credited. It is in such that the outer coats of these vessels are most happily of all to be examined.
THE METHOD OF PREPARING SALTS AND SALINE SUBSTANCES, FOR THE VIEWING THEIR CONFIGURATIONS.
Dissolve the subject to be examined in no larger a quantity of river or rain water than is sufficient to saturate it; if it be a body easily dissoluble, make use of cold water, otherwise make the water warm or hot, or even boiling, according as you find it necessary. After it is perfectly dissolved, let it rest for some hours, till, if over-charged, the redundant saline particles are precipitated, and settle at the bottom, or shoot into crystals; by which means you are most likely to have a solution of the same strength at one time as at another; that is, a solution fully charged with as much as it can hold up, and no more; and by these precautions the configurations appear alike, how often soever tried: whereas, if the water be less saturated, the proportions, at different times, will be subject to more uncertainty; and if it be examined before such separation and precipitation of the redundant salts, little more will be seen than a confused mass of crystals.
The solution being thus prepared, take up a drop of it with a goose quill, cut in fashion of a scoop, and place it on a flat slip of glass, of about three quarters of an inch in width, and between three and four inches long, spreading it on the glass with the quill, in either a round or oval figure, till it appears a quarter of an inch or more in diameter, and so shallow as to rise very little above the surface of the glass. When it is so disposed, hold it as level as you can over the clear part of a fire that is not too fierce, or over the flame of a candle, at a distance proportionable to the degree of heat it requires, which experience only can direct, and watch it very carefully till you discover the saline particles beginning to gather and look white, or of some other colour, at the extremities of the edges; then having adjusted the microscope before-hand for its reception, armed with the fourth glass, which is the fittest for most of these experiments, place it under your eye, and bring it exactly to the focus of the magnifier; and after running over the whole drop, fix your attention on that side where you observe any increase or pushing forwards of crystalline matter from the circumference towards the center.
This motion is extremely slow at the beginning, unless the drop has been over-heated, but quickens as the water evaporates, and in many kinds, towards the conclusion, produces configurations with a swiftness inconceivable, composed of an infinity of parts, which are adjusted to each other with an elegance, regularity, and order, beyond what the exactest pencil in the world, guided by the ruler and compass, can ever equal, or the most luxurious imagination fancy.
When action once begins, the eye cannot be taken off, even for a moment, without losing something worth observation; for the figures alter every instant, till the whole process is over; and in many sorts, after all seems at an end, new forms arise, different entirely from any that appeared before, and which probably are owing to some small quantity of salt of another kind, which the other separates from, and leaves to act after itself has done; and in some subjects three or four different sorts are observable, few or none being simple and homogeneous.