metal is dissolved in aqua fortis, and again reduced to a solid form, it appears as a white powder, and will preserve its colour if carefully kept from the light; but if a vial be filled with it, and exposed to the sun, that side on which the light falls will in a short time become black, and this though the vial has been ever so carefully sealed.[71] Formerly, chemists had a method of accounting for this appearance, as well as that of the venous blood, by what they called the _evolution of phlogiston_: but now that the very existence of phlogiston is denied, we are deprived of this resource. But, whatever words we may use, it is plain that in neither case have we any ideas affixed to them which can make the matter at all more intelligible than it was before. But with regard to the blood, we are at a considerable loss to understand what the natural colour of it is; and indeed the question can only be determined by examining the blood of a fœtus which has never breathed. If the arterial blood of such a fœtus be of a dark colour, resembling that in the veins of a grown person, we must look upon this to be _natural_ to it, and we may as well inquire why a rose is red, or an iris blue, as why the blood is of a dark, and not of a bright red. But, if we find this dark red change to a bright scarlet in the arteries, as soon as the child has breathed, we have as much reason to conclude that the air occasions this superior redness, as that an acid is the cause of a red colour in the syrup of violets, or an alkali of a green colour in the same. Experiments are yet wanting to determine this matter. Mr. Hunter has observed that “in such fœtuses as convert animal matter into nourishment, they most probably have it (the colour of the blood) influenced by the air, such as the chick in the egg, although not by means of the lungs of the chick, we find the blood, in the veins of their temporary lungs, of a florid colour, while it is dark in the arteries.”--The probability therefore is, that the blood is naturally dark; by the elastic principle of the oxygen that it is rendered brighter; and that, this elastic principle being expended in the course of circulation, the fluid reassumes its original colour. [70] A glass tube is sealed hermetically, by heating the open end or ends, till they become soft, and then closing them with a pair of pincers. [71] Thus letters, or other characters, may be curiously marked upon the calx within the vial, by cutting them out in paper, and then pasting them on the side to be exposed to the light. We may have them in this manner either dark upon a white ground, or white upon a dark ground.