CHAPTER XIII.

THE FIRST SUN-PICTURES.

Humphry remained confined to his bed for many weeks, and every day his mother dreaded seeing some development of the symptoms which generally ensue from the bite of so rabid an animal; but the boy assured her, again and again, that, owing to the promptitude of the remedies he had applied, there was no cause for alarm.

While the youth was still a prisoner in his room, a gentleman came to lodge at the house, and Humphry, to his great delight, learnt, in a few days afterwards, that the new lodger was no less a person than Mr. Gregory Watt, son of the celebrated James Watt, the inventor of the present steam-engine.

It did not take long before the two became acquainted, and then Humphry ascertained that the young Mr. Watt had but recently quitted the University of Glasgow, and had been recommended by his physicians, owing to his declining state of health, to reside for some time in the West of England; hence the cause of his visit to Penzance.

Nor was Humphry long in discovering that the mind of his new friend was enriched beyond his age with science and literature, and that he possessed, like himself, a spirit devoted to the acquisition of knowledge, and soaring far above the little vanities and distinctions of the world. The two kindred spirits, therefore, soon contracted an intimacy of the warmest nature; and this ultimately ripened into a friendship which continued to the period of Mr. Watt’s premature dissolution.

Mr. Gregory Watt felt not a little astonished, on being introduced to the son of his landlady, to find him shortly afterwards speaking upon subjects of metaphysics and poetry; for when Mr. Watt spoke to him of the courage he had displayed at the time of the accident in excising the wounded parts of his leg, Humphry confessed “that he had no belief in the existence of pain, whenever the energies of the mind were directed to counteract it.”[52] “For,” went on the boy, “in states of profound attention, all perception of external things fades from the mind; the clock in the room ticks, and we hear it not; persons enter the apartment, and their presence is unheeded by us. And if by the intensity of the intellectual operations the senses can cease performing their functions in the one case, why not in the other? Martyrs at the stake have, while in deep prayer, held their hand unmoved in the flames, and who can say that the very fervour of their heavenly aspirations did not deprive them of all sense of pain for the time being?”

Mr. Gregory Watt, however, had but little taste for metaphysical discussions; and, smiling at Humphry’s stoicism, sought to divert the conversation into a more congenial and practical channel.

The steam-engine that had been recently set up at the Wherry Mine by Mr. Watt’s father became the theme of their converse; this soon led to comments on the theory upon which its powers depended, and then Humphry’s companion was surprised to find that a youth, who had been brought up in an obscure town in Cornwall, was as well acquainted with the doctrine of “latent heat”—and, indeed, the whole science of caloric—as he himself, who had been reared, as it were, in a factory, where the workings of it were every day visible.

The new laws of combustion naturally followed as the next subject of discussion, when Humphry observed, “that he would undertake to demolish the French theory in half-an-hour;” and so saying, he rapidly ran over the experiments he had performed, to prove the falsity of Lavoisier’s notions respecting the origin of heat during the burning of substances.[53]

The lad had now touched the true chord, and the interest of Mr. Watt becoming more excited, he conversed with young Davy upon his chemical pursuits, and was at once astonished and delighted at his sagacity; so that the couple—congenial in taste—already began to feel a growing friendship for each other.

* * * * *

Humphry had quitted his chamber, but was still confined to the house, from his inability to walk, when Mr. Watt—who had now known the youth long enough to be proud of his acquaintance—returned from his morning’s stroll along the sea-shore, in company with two friends whom he had met in the town.

The gentlemen proved to be Mr. Josiah Wedgwood, the eminent potter of Staffordshire, and his brother Thomas, who was alike distinguished for his scientific abilities.

The learned potter was not long in Humphry’s company before he discovered the high merits of the lad; nor was he a little pleased when he found that the young Cornish apothecary knew all about the pyrometer he had invented for measuring high degrees of heat by the contraction of a ball of clay; and the old gentleman found considerable delight in explaining to the boy the various processes concerned in the manufacture of earthenware and porcelain, telling him anecdotes as to how Bernard Palissy—who was the first to discover the means of giving a glaze to the baked clay—had been reduced to such poverty by his experiments, that he was forced to burn the doors, and even the boards, of the house in which he lived, in order to get a supply of fuel for his furnaces; and how he afterwards amassed an immense fortune by the invention, and ultimately died in the French Bastille, a martyr to the Protestant creed, which he had espoused. Mr. Wedgwood added that, “When we eat our food we little think of the labour and privations that have been endured in order to give a glassy surface to the plates and dishes upon which it is served; for but few are aware that, previous to this invention, the ordinary earthenware articles were more like tiles than our present crockery.”

Then Mr. Wedgwood talked with the youth about the rocks, inquiring whether he had ever noticed any of the finer species of clay in those parts, and was surprised to find how closely the boy had marked the changes in the soil; for Humphry told him “he had observed that the felspar in the granite decomposed long before either the mica or the quartz, and that it was chiefly by the action of the atmosphere upon this same felspar that the huge granite rocks became disintegrated, or broken up; and that, as the felspar consisted principally of clay in the purest form, he fancied that some advantage might be taken of this in producing a finer species of porcelain than had yet been manufactured in this country.”

The old gentleman thanked Humphry for his suggestion, and warmly praised him for his observation and sagacity; whereupon the youth promised, immediately that his leg would permit of his accompanying him, to point out to Mr. Wedgwood the places where he had noticed the finest deposits of felspar to occur.

The conversation then changed to a subject that Mr. Josiah Wedgwood said “he had no doubt would be highly interesting to one of Humphry’s turn of mind.” The old gentleman told the boy how his brother Thomas had discovered a means of copying pictures upon glass, and even of fixing the images of the _camera obscura_, by the action of light; “so that,” he said, “the sun itself could be made to turn artist, and to produce a representation of an object that no human hand could possibly rival.”

Humphry was enraptured with the new wonder, and more eager than ever to learn all about it; so he begged Mr. Thomas Wedgwood to explain to him the whole process.

“I should tell you, then,” said the potter’s brother, “that it has been long known to chemists that a solution of _nitrate of silver_ (called _lunar caustic_ in the shops), when washed over a sheet of paper—although it does not undergo any change while kept in a dark place—will speedily change colour on being exposed to daylight, and that then it passes through different shades of grey and brown, and ultimately becomes nearly black. These alterations in colour,” continued Mr. Thomas Wedgwood, “take place more rapidly according as the light is more intense. In the direct beams of the sun, two or three minutes are sufficient to cause it to darken; whereas in the shade, several hours are required to produce the full effect. The light, too, when transmitted through different-coloured glasses, acts upon the nitrate of silver with different degrees of intensity. It is found, for instance, that the sunbeams, when passed through red or yellow glass (so that only red or yellow light shall fall upon the paper), have very little action upon the lunar caustic; green glass, however, is more efficacious, while blue and violet produce the most decided and rapid changes.

“Now to make you clearly comprehend,” went on the gentleman, “the reason of these changes, I should tell you that nitrate of silver consists of nitric acid (aquafortis) and silver,[54] and that if a vessel of pure and colourless nitric acid be exposed to the sun’s rays, it will become decomposed, so that red fumes of nitrous acid will be evolved, and these mixing partly with the liquid itself, will shortly turn it to a reddish-brown tint. Well, the same change takes place when the nitric acid is combined with the silver, and so made to form nitrate of silver. The consequence is, that as the nitrous acid which is evolved is unable to combine directly with the silver, the decomposed aquafortis is dissipated in the form of vapour, while the silver itself remains behind in the paper; and in such an extreme state of minute division, that the particles, instead of being white, like ordinary silver, appear black to our eyes.”

Humphry expressed himself delighted with the explanation, and said he could now see how it was possible to produce sun-pictures by such means. Still he begged Mr. Wedgwood to proceed.

That gentleman then told Humphry that his first attempt concerning the production of sun-pictures, was to fix the evanescent images formed by the camera obscura, but though he was able to impress these upon paper in a bright sunlight, he found that he could not produce them in any moderate time in ordinary daylight; so that, from the length of time required before the impression was taken, the effects of light and shade had materially altered. “With paintings on glass, however,” he added, “I have been more successful; in order to copy these I apply the solution of nitrate of silver to leather, for this I find to be more readily acted upon than paper—probably owing to the tanning in the material. When the surface of leather is thus prepared, I place it behind a painting upon glass, and expose it to the solar light, when the rays, being transmitted through the different parts of the picture, produce distinct gradations of black and white, according to the lights and shades in the original; for where the light passes freely through the glass, the colour of the nitrate of silver, of course, becomes the deepest. By this means, then, you will perceive that the lights and shades of my picture are entirely reversed: all the black parts in the original being left white in the copy, since the light, being unable to pass through these, cannot act upon the solution; while all the white parts of the original, on the other hand, become the blackest in the copy, owing to the rays passing freely through the glass there, and so producing the strongest effect. Accordingly, I am obliged to have the original pictures painted, in the first instance, with their lights and shades reversed, or else I cover another glass with a thin coating of isinglass, and apply the solution of nitrate of silver to this; so that, when I have transferred the original picture by this means to a second plate of glass, in which the lights and shades are the direct opposite to what they are in nature, I proceed to take a second copy of it—but this I do upon leather, as before explained—and so obtain a perfect reproduction of the original, with all the lights and shades in their proper places.

“But the same method of copying,” proceeded Mr. Wedgwood, “may be applied to other purposes. It may be rendered subservient, for instance, for making delineations of all such objects as are partly opaque and partly transparent, such as leaves and the wings of insects. For this purpose it is only necessary to put the objects to be copied between a plate of glass and the prepared leather itself, when the sunlight, being more or less intercepted by their forms, will leave the figures accurately impressed upon the leather, so that they will appear as beautiful white pictures upon a black ground. There is, however,” added Mr. Thomas Wedgwood, “one great defect connected with the production of sun-pictures by the means I have described, and this consists in the impossibility of fixing them so that they shall be no longer susceptible of being darkened when exposed to the light. I have already tried several methods of obviating this difficulty. I have covered the sun-pictures with a thin coating of varnish, but to no purpose, for they darken almost as rapidly with the varnish over them as others do without it. Again, I have submitted the pictures to frequent washings, in the hopes of dissolving out of the paper or leather all the undecomposed nitrate of silver; yet, even after this, a certain portion of the active matter still adheres to the white parts of the sun-picture, and so causes them to blacken all over on being exposed to the light. The consequence is, that the pictures produced by the action of the sun must, in order to be preserved, be examined always in the dark, and be kept continually in some place where no light can penetrate.”

Humphry no sooner heard this than he suggested a number of expedients by which he fancied the difficulty might be overcome; and as the lad explained his reasons for the various methods he proposed, both Mr. Wedgwood and his brother were as astonished at the extent of the boy’s knowledge, as they were delighted with the acuteness of his sagacity.

The evening was passed in examining a portfolio of the sun-pictures that Mr. Thomas Wedgwood had brought with him, and Humphry grew so charmed with the then entirely novel process of “_photography_” that he declared he would not rest until he had investigated the matter himself, and ascertained experimentally whether any means could be found of rendering the pictures permanent.