will be then as flow as can well be perceived, and the space defcribed by it upon the floor, will be about 10 inches in an hour; and the motion of the image of the fhade, upon the retina of an eye looking at it at the distance of 11 inches, will be at the rate of about of an inch in an hour, or.01 of an inch in a minute. By the above fuppofition, the space described in an hour by an object having the floweft perceivable motion, and the distance of the eye from the object, are nearly equal: and therefore the space described in a minute of time, by an object having fuch a degree of motion, fubtends an angle at the eye of about one degree or the fpace defcribed in a min. in a line perpendicular to the optic axis, by an object having the leaft perceivable motion, is about of the distance between the object and the spectator. Hence, we fee why the heavenly bodies are not perceived to move; the spaces defcribed by them in a minute, not fubtending an angle of above a degree, when their apparent motion is greatest. It is difficult to align the greatest angular space, that can be defcribed in a given time, by an object having the greatest perceptible motion; and the difference may be very great, betwixt that degree of velocity with which fomething may be perceived to cross the optic axis, and that, with which the fame object may be diftin&tly feen, as it moves of this latter fort, let that of a falling body at the distance of 20 feet from the fpectator, and in that part of its fall when its velocity is at the rate of 19 feet in a fecond, be the quickeft perceivable motion; the motion of the image upon the retina at that inftant, will be at the rate of about of an inch in a fecond; and therefore upon this fuppofition, the quickest perceivable motion is to the floweft, as an hour is to a fecond of time, or as about 3600 to 1. We do not here pretend to affign the precife limits of either the leat or greatest perceivable velocity; what has been observed above, is fufficient to fhew that they have their limits, and perhaps thefe limits are not far from those above supposed; and perhaps alfo, the ratio of the greatest to the flowell of velocities upon the retina, that are diftinctly perceivable, is not greater than about 2000 to 1. We have feen above, that a prodigious great velocity, fuch as is the diurnal motions of the heavenly bodies, may be yet too flow to be perceived; and the flight of a ball out of a gun, is much too quick for an eye not far off' From other experiments and confiderations alfo, we have reafon to think the above to be near the limits of vifible motion. The cafe of the floweft vifible motion any perfon may eafily illuftrate by the minute-index of a common watch; for if that index be about an inch in length, the end of it will defcribe about 6 inches in an hour; and if a perfon look VOL. XLI. April, 1776. fteadily X steadily at it at the distance of 6 inches from it, he will be able just to perceive it move; and then it plainly defcribes a space equal to the distance of the eye in an hour, or one 60th of that distance in a minute, as in the above extract from our author. Towards the conclufion of this fection on vifion, he has added a short difcourfe on fquinting. But our limits will not admit of our extracting from all the curious articles that we meet with in this valuable work. Our author next proceeds to treat of the minimum vifibile, or the leaft angle under which objects are visible, which is certainly various according to their different degrees of luminoufnefs; of diftinct and indistinct vision; of long and fhort fighted eyes, and of fpectacles; alfo of apparent diftances, and magnitudes; and of vifion by images; and enumerates the accidents by which long and short-fightedness happen, the means of cure, and proper use and choice of fpectacles and reading-glaffes. His reafoning on apparent and real distances and magnitudes, both celestial and terrestrial, is full and fatisfactory. We are then led to the confideration of vifion by light reflected from plane and fpherical fpeculums; of vision through given mediums; of pictures feen through convex ¡enses; and a defcription of different forts of camera-obfcuras. Among thefe the following are very curious and interesting, viz. obfervations and directions relating to the judgment and choice of plane and curved fpeculums; many curious contrivances for difpofing pictures, fcenes, &c. in boxes, with mirrors and lenfes fecretly and properly placed, to produce furprising effects the deferiptions of various inftruments for viewing prints, and improving them: the finding of the focal lengths of lenfes by various experiments difcourfe on the means of affifting divers to fee under water: the application of camera-obfcuras to little boxes for the purposes of drawing; alfo various fhew-boxes, &c. This Treatife will be useful to all who have occafion to read or carry into practice the fcience of optics. And it is to be regretted, that the author did not live to complete the remaining part of the work. IX. Efays Phyfical and Chemical, by M. Lavoifier. Tranflated from the French, with Notes, and an Appendix, by Thomas Henry, F. R. S. 8vo. 7. Johnfon. M Lavoifier of Paris, a gentleman of diftinguished rank, and an intendant of the finances, is one of thofe experimental philofophers who have fignalized thenfelves on the con continent, by profecuting the researches in chemistry, which have lately been made with fo much fuccefs by fome ingenious inquiters in this country. Dr. Priestley's discoveries, in particular, refpecting air, have excited the attention and exercised the investigation of all who are zealous for the improvement of natural knowledge. The doctrine of fixed air, however, which he feems to have fo clearly established, has met with fome opponents in Germany, who have fubftituted in its room another agent under the name of acidum pingue. According to Dr. Black's fyftem, the caufticity of alcalis and quick-lime, and the folubility of the latter in water, depend on their being deprived of fixed air; but the German philofophers maintain that these properties refult from those bodies poffeffing the acidum pingue. To decide a controverfy of fo great importance to science, has been M. Lavoifier's fole motive for profecuting the experiments in this Effay; the tranflation of which, we are perfuaded, cannot fail of being acceptable to English readers; especially as Mr. Henry has corrected many paffages, in which his author appears to have miftaken the meaning of Dr. Priestley. The first part of this volume contains a hiftorical account of of what has hitherto been done, relative to the combination of air with bodies; with an accurate defcription of the various discoveries on this fubject. The fecond part commences with the experiments made by the author, toward fixing the opinion of chemifts in regard to the different fyftems of Dr. Black and Mr. Meyer. After enquiring into the exiftence of an elaftic fixable fluid in calcareous earths, and the phenomena refulting from the abfence of it in lime, M. Lavoifier deduces the following obfervations, as neceflary consequences. First, That there exifts in calcareous ftones and earths, an elaftic fluid, a fpecies of air under a fixed form, and that this air, when it has recovered its elafticity, poffeffes the principal physical properties of air. Secondly, That a hundred pounds weight of chalk, according to the above proportions, contains about thirty-one pounds, fifteen ounces of this elaftic fluid; fifteen pounds, feven ounces of water; and only fifty-two pounds, ten ounces of alkaline earth. Thirdly, That it is even poffible, that the chalk may contain ftill lefs alkaline earth, and more elaftic fluid, but that hitherto we are not acquainted with any method of depriving it of more, or of carrying its analyfis farther. Fourthly, That alkaline earth may exift in three different flates: firft, faturated with water and elaftic fluid, as in chalk : fecondly, deprived of its elastic fluid, but faturated with water, as in flaked lime: and, thirdly, deprived both of its elastic fluid and water as in quick-lime. Fifthly, That quick lime, or alkaline earth, deprived both of its water and elastic fluid, contains a great quantity of the matter of pure fire, which it has probably acquired during its calcination, and that to this matter is owing the great heat which is obfervable during the extinction of lime, and its diffolution in acids. Sixthly, That it is not fufficient to faturate quick lime with water, in order to deprive it of the fuperabundant quantity of igneous particles: but that they remain after this operation; fince flaked lime communicates a confiderable degree of heat to the nitrous acid, in which it is diffolved; a phenomenon which is not produced by calcareous earth or chalk." Seventhly, That it is by no means this fuperabundant ig neous matter which reduces the alkaline earth into the state of lime, fince flaked lime, when deprived by the flaking of a great part of this fire, is, notwithstanding, no lefs foluble in water, ftill continues to decompofe fal ammoniac without the affistance of heat, and does not communicate a lefs degree of caufticity, to either the fixed or volatile alkalis. In a word, it is no lefs lime than before it has been flaked. • Laftly, That it is fufficient that we reftore to lime, by any means whatsoever, the elaftic fluid of which it has been deprived, to render it mild, infoluble in water, and capable of effervefcing with acids; in fhort, to re-establish it in the state of calcareous earth or chalk.' The multiplicity of experiments which the work contains not admitting of a particular detail, we can only inform our readers of the subjects on which they are made. In the fubfequent chapters, therefore, the author treats of the Existence of an elaftic fixable Fluid in the fixed and volatile Alkalis, and of the Means by which they may be deprived of it-Of the Precipitation of calcareous Earth, diffolved in nitrous Acid, by Alkalis with metallic Subftances by Precipitation-Of the Existence of elaftic fixable Fluid in the metallic Calces-Of the Combination of elastic Fluid with metallic Subftances by Calcination -Experiments on elaftic Fluid difengaged from effervefcent Mixtures, and from metallic Reductions-Of fome Properties of Water impregnated with elaftic Fluid feparated from effervefcing Mixtures, or metallic Reductions-On the burning of Phofphorus, and the formation of its Acid-Experiments on Combustion and Detonation in vacuo-Of Air in which Phofphorus has been burnt. An Appendix is added by the tranflator, containing a Memoir of M. Lavoifier's, read before the Royal Academy of Sciences, fciences, on the nature of the principle which combines with metals during their calcination and increases their weight; and an account of Dr. Prieftley's opinion relative to the principle which is combined with metals during their calcination, and of his discovery of dephlogisticated air. M. Lavoisier proposes that this volume fhall be followed by feveral others, and he gives a general account of the subjects of which he intends to treat. As he is an ingenious and accurate experimentalift, we hope that his laudable enquiries will contribute to the farther afcertainment of many principles of natural philofophy, for the cultivation of which he appears to be peculiarly qualified. X. An Effay on the Water commonly used in Diet at Bath. THAT HAT the falubrity of water used in diet is a matter of the utmost importance, to health, has been univerfally acknowledged by physicians from the days of Hippocrates. An obvious diversity in the fenfible qualities of different waters, joined to medical obfervations on thofe by whom they were drunk, were fufficient to fuggeft the idea of their exerting great influence on the conftitution. But fince chemistry began to be cultivated, men have discovered more fcientific criteria, by which the good or bad effects of water may now be determined a priori, without the neceffity of examining their actual operation. Within thefe few years, Dr. Heberden in London, and Dr. Percival at Manchester, have favoured the public with ufeful obfervations on the water used in these places; and it is to be wifhed that the gentlemen of the faculty would profecute the enquiry in other parts; especially where different kinds of water may be procured, and the health of the inhabitants confequently benefited by the use of that which deferves the preference. It appears that this treatife was undertaken at the defire of Dr. Fothergill, who had recommended to the author an inveftigation of the dietetical water at Bath; probably on account of the accuracy and induftry which Dr. Falconer formerly discovered, in his obfervations on the medicinal waters of that place. The Effay is divided into two parts; in the first of which the author treats of the feveral forts of water which are used in dier, fuch as rain-water, river-water, fpring-water, &c. and mentions various fubftances by the mixture of which with water, the properties of the latter may be known. In the fecond part of the Effay, the author particularly examines the X 3 fpe |