ART. VIII.

Philosophical Transactions of the Royal Society of London. For the Year 1813. Parts. I. and II. 4to. 148. and 18s. Nicol and Son.

CHEMICAL and PHYSIOLOGICAL Papers. Part I.

ON a new detonating Compound, in a Letter from Sir H. Davy, LL.D., F.R.S., to Sir Joseph Banks, Bart., P.R.S.-During the autumn of 1812, Sir H. Davy received, from a correspondent in Paris, an account of a new detonating compound, of extraordinary violence, which had been discovered in that city; and which was said to be a mixture of chlorine and azote, and to assume the appearance of a heavy oil: but nothing was stated respecting the manner in which it was to be formed, The author immediately instituted a variety of processes for the purpose of procuring it; and he succeeded so far as to obtain an oily substance, which exploded violently on the application of heat, by exposing a quantity of chlorine to a saturated solution of the nitrate of ammonia. The explosion was so strong as to have produced a serious accident to the operator; and one motive, which induced Sir H. Davy to publish an account of the investigation in so early a stage of it, was, as he informs us, to put other chemists on their guard against its dangerous effects. The substance is described as resembling olive-oil in its colour and appearance, but having much greater specific gravity; its smell is nauseous, and it is extremely pungent. It acts with violence on both the expressed and the essential oils; ether and alcohol act on it more slowly; it is decomposed by phosphorus under water, converts mercury into corrosive sublimate, but has not much effect on tin, zinc, sulphur, or resin. The following observations exhibit the author's opinion of its nature, and the mode of its production; which, it is almost unnecessary to remark, proceeds on his new hypothesis respecting the composition of the muriatic and the oxymuriatic acids:

It seems probable, from the general tenor of these facts, that the new substance is a compound of azote and chlorine; the same as, or analogous to, that mentioned in the letter from Paris. It is easy to explain its production in our experiments: the hydrogen of the ammonia may be conceived to combine with one portion of the chlorine to form muriatic acid, and the azote to unite with another por tion of chlorine to form the new compound. The heat and light produced during its expansion into gaseous matter, supposing it to be composed of azote and chlorine, is without any parallel instance, in our present collection of chemical facts; the decomposition of cuchlorine, which has been compared to it, is merely an expansion of matter already gaseous. The heat and light produced by its rare. faction, in consequence of decomposition, depend, probably, on the

F 3

same

same cause as that which produces the flash of light in the discharge of the air-gun.

The mechanical force of this compound in detonation, seems to be superior to that of any other known, not even excepting the ammoniacal fulminating silver. The velocity of its action appears to be likewise greater.'

Observations relative to the near and distant Sight of different Persons. By James Ware, Esq., F.R.S. It is here observed by this experienced and respectable practitioner, that the state of the eyes which produces short-sightedness seldom exists at birth, but generally comes on between the ages of 10 and 18, when it is at first noticed accidentally. In the lower ranks of life, in which persons are usually engaged in such occupations as lead them to exercise their eyes on distant objects, short-sightedness is a rare occurrence; and when it does take place, as they seldom have recourse to the use of glasses, it is probably overcome by the increased exertion which the eyes make to enable them to view objects that are remote. The higher ranks, however, immediately apply to the convex lens, and frequently to one of a higher power than is necessary to correct the defect; so that the state of the eye becomes confirmed, or even increased. The subsequent observations seem to prove the truth of the author's conclusions:

With regard to the proportion between the number of nearsighted persons in the different ranks of society, I have taken pains to obtain satisfactory information, by making inquiry in those places where a large number in these several classes are associated together. I have inquired, for instance, of the surgeons of the three regiments of foot guards, which consist of nearly ten thousand men; and the result has been, that near-sightedness, among the privates, is almost utterly unknown. Not half a dozen men have been discharged, nor half a dozen recruits rejected, on account of this imperfection, in the space of nearly twenty years: and yet many parts of a soldier's duty require him to have a tolerably correct view of distant objects; as of the movements of the fugleman in exercise, and of the bull's eye when shooting at the target; the want of which might furnish a plausible apology for a skulker to skreen himself from duty, or to get his discharge from the service. I pursued my inquiries at the military school at Chelsea, where there are thirteen hundred children, and I found that the complaint of near-sightedness had never been made among them until I mentioned it; and there were then only three who experienced the least inconvenience from it. After this, I inquired at several of the colleges in Oxford and Cambridge; and, though there is a great diversity in the number of students who make use of glasses in the various colleges, they are used by a considerable proportion of the whole number in both Universities; and, in one college in Oxford, I have a list of the names of not less than thirtytwo out of one hundred and twenty-seven, who wore either a hand glass or spectacles, between the years 1803 and 1807.'

The

The practical inference from these facts is sufficiently obvious and important; and Mr. Ware particularly remarks that it is very desirable for those who feel it necessary to employ a concave lens, to use a number a little lower than that which gives them the most distinct vision; since this will answer nearly as well for all the common purposes of life, and will tend rather to diminish than to increase the defect. Instances are mentioned in which short-sightedness took place in infancy, or in early childhood, and sometimes this is evidently the effect of disease; and a case is reported in which the eye was suddenly rendered near-sighted from some morbid affection of the brain, which terminated in the death of the patient. The degree of near-sightedness is seldom exactly the same in both eyes. Mr. Ware does not think that the pupils of near-sighted persons are more dilated than those of others.

Dr. Wells, in a late paper in the Philosophical Transactions, related some experiments on the effect which the permanent dilatation of the pupil, arising from the application of the extract of belladonna, produced in preventing the distinct view of near objects. Mr. Ware has repeated the experiment with the same result. In a case which he examined, the nearest point of distinct vision was removed from 4 to 7 inches; while the farther point, at which a small object could be distinctly seen, was also removed from 31 to 34 inches. The same alteration in the limits of distinct vision is generally effected in the eyes of those who are near-sighted, by the permanent dilatation of the pupil: but, in some few instances, the opposite result takes place.

It is known that the change in the state of the eyes, as life advances, consists in a loss of the power of seeing near objects correctly, while such as are remote are still seen distinctly but sometimes the power of viewing both near and remote objects is equally lost; and, as the same lens corrects both defects, it shews that the eye retains its usual power of adjusting itself to different distances. In this respect, it is said to differ from the eye which has lost its crystalline humor, to which a different lens is necessary for viewing near and discerning remote objects. Mr. Ware, who is himself near-sighted, remarks that such persons have generally a less extensive range of vision than those who have a distant sight; and, contrary to the common opinion, he conceives that their eyes are disposed to become more near-sighted as life advances.

Some of the more important points, which Mr. Ware attempts to establish in this valuable paper, are comprized in the ensuing positions, with which it concludes:

First; near-sightedness is rarely observed in infants, or even in children under ten years of age. It affects the higher classes of society more than the lower: and the instances are few, if any, in which,

if the use of concave glasses has been adopted, increasing years have either removed or lessened this imperfection.

Secondly; though the usual effect of time on perfect eyes be that of inducing a necessity to make use of convex glasses, in order to see near objects distinctly, yet sometimes, even after the age of fifty, and after convex glasses have been used many years for this purpose, the eyes have not only ceased to derive benefit from them, when looking at near objects, but they have required concave glasses to enable them to distinguish, with precision, objects at a distance,

Thirdly; though the cause of this change be not always known, yet sometimes it has been induced by the use of evacuating remedies, particularly of leaches applied to the temples; and sometimes by looking through a microscope, for a continued length of time, in several successive days.

Fourthly; instances are not uncommon, in which persons, far advanced in life, (viz. between eighty and ninety,) whose eyes have been accustomed for a long time to the use of deeply convex glasses, when they have read or written, have ceased to derive benefit from these glasses, and they have become able, without any assistance, to see both near and distant objects almost as well as when they were young. Although it be not easy to ascertain the cause of this amended vision, it seems not improbable that it is occasioned by an absorption of part of the vitreous humour; in consequence of which the sides of the eye collapse, and its axis, from the cornea to the retina, is lengthened; by which alteration the length of this axis is brought into the same proportion to the flattened state of the cornea or crystalline, or both, which it had to these parts before the alteration took place.'

The Bakerian Lecture. On the Elementary Particles of certain Crystals. By W. H. Wollaston, M. D., Sec. R. S.-It has always been considered as a question difficult of solution, to what primitive form the regular crystallized octohedron ought to be referred; because, though the tetrahedron might seem to be its most natural origin, as being most readily converted into an octohedron, yet it is in fact not the tetrahedron which we procure in our experiments. If fluor spar be employed, it is separated into rhomboids; and each of these rhomboids may again be divided into one octohedron and two tetrahedrons. It is therefore not easy to determine which of these figures should be regarded as primitive; and, since neither tetrahedrons nor octohedrons can occupy space without leaving vacuities, to shew how these figures can form the basis of a permanent crystal. To remove these difficulties, Dr. Wollaston was led to form an hypothesis, which appears to be both ingenious and satisfactory. With respect to fluor spar, (he says,) and such other substances as assume the octohedral and tetrahedral forms, all difficulty is removed by supposing the elementary particles to be perfect spheres, which by mutual attraction have assumed that arrange

ment which brings them as near to each other as possible.' He then proceeds to illustrate his new doctrine, by shewing what figures would be produced by the different combinations of spheres, from the equilateral triangle to the regular tetrahedron or the regular octohedron. If we suppose the particles, instead of being regular spheres, to be flattened, we have a rhom boid; and, by conceiving them to be rendered oblong instead of oblate, we may compose an hexagonal prism. - Dr. W. informs us that an hypothesis, similar to his own, respecting the spherical shape of the particles of crystals, was suggested by Dr. Hooke, but appears to have been entirely neglected.

On a Substance from the Elm Tree, called Ülmin. By James Smithson, Esq., F.R.S. A peculiar exudation flows from the Elm-trée, which was originally examined by Klaproth, and to which Dr. Thomson applied the name of ulmin, supposing it to be a distinct vegetable principle. Mr. Smithson, however, made some new experiments on it, and concludes that it is not a simple vegetable substance, but that it consists of a combination of potash and a matter which is analogous to the extracts. He calculates that ulmin contains from 15 to 20 per cent. of potash.

On a Method of Freezing, at a Distance. By W. H. Wollaston, M.D., Sec. R.S.It is well known that, when a portion of a fluid is evaporated, the remainder becomes colder in consequence of the obstruction of heat by the vapor; and also that fluids rise in vapor at a lower temperature in vacuo than in the open air. Water may be frozen by means of the vacuum of an air-pump, but it is necessary that the pump be in the best condition, that the receiver be large, and the quantity of water very small. Our philosophical readers no doubt are aware that Professor Leslie has discovered a very ingenious method of expediting the freezing of water in a vacuum, by employing sulphuric acid; which, in consequence of its attraction for water, absorbs the vapor as fast as it is formed, and thus preserves the vacuum in its perfect state. Dr. Wollaston's mode is more simple, and requires a less expensive apparatus. It consists of a tube, with a ball at each end, bent at right angles; one of the balls is to contain a little water; and the remainder of the space is to be as perfect a vacuum as can be obtained. Its operation is thus described:

When an instrument of this description has been successfully exhausted, if the ball that is empty be immersed in a freezing mixture of salt and snow, the water in the other ball, though at the distance of two or three feet, will be frozen solid in the course of a very few minutes. The vapour contained in the empty ball is condensed by the common operation of cold, and the vacuum produced by