this curious memorial, which, to have its due influence on the reader's mind, must be perused at large, to the attentive confideration of the public.

As in northern latitudes fheep are exposed to great diftress in deep fnows, the paper, No. VII. explains the ufe of what is called the fnow-plough, to clear grafs and turnips, at such seafons, fo as to be acceffible to the sheep; and alfo to render the roads paffable.

FOREIGN LITERATURE.

ART. XXI. Nouvelles Experiences et Obfervations fur divers Objets de Phyfique, &c. i. e. New Experiments and Obfervations on various Branches of Natural Philofophy. By J. INGENHOUSZ, Aulic Councellor, Phyfician to his Imperial Majefty, Fellow of the Royal Society, &c. &c. &c. Vol. II. Large Octavo. PP. 574. Paris. 1789.

THE

HE celebrity of M. INGENHOUSZ, so justly acquired, and the light which his former publications have diffufed over fome parts of natural philofophy, that had lain unexplored, are sufficient guarantees for the acceptance of this prefent collection of obfervations, which the philofophic world have been long expecting with a degree of impatience. The author informs us, in his preface, that this work is to be confidered as a continuation of that tranflated from the German into the French language, under the title of Mélange de Phyfique & de Médecine; although by an unfortunate blunder, or rather, as it appears, by the officioufnefs of the printer in his abfence, a new title is prefixed which circumftance has prevented his adding fuch articles as were purely medical. Some articles of its contents alfo correfpond with the treatife published feveral years ago, under the title of Experiments on Vegetables; of which an ample account has been given in our 62d volume, p. 345.

The publication before us is rich in materials. It contains new obfervations on vegetables, airs, electricity, chemistry, mineralogy, &c. &c. which are all treated with the fpirit of obfervation that characterizes a genuine philofopher. As a minute review of each article would be impracticable, we shall felect fuch specimens as promife to be moft acceptable to our philofophic readers; and content ourfelves with announcing the fentiments of M. INGENHOUsz on the other subjects.

The work commences with a fmall effay on the use of microfcopes; in which the author, after expatiating on the extreme difficulty attending the examination of animalcula in a Auid medium, arifing from the refraction of light, and the - REV. OCT. 1790.

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evaporation of the fluid, recommends, as the best expedient, placing a small thin lamella of glafs on the drop to be examined; which renders its furface more uniform, and retards evaporation. Trifling as this circumftance may feem, the experience of this philofopher teftifies its great utility.

The first memoir contains a minute examination of that green matter obfervable on the surface of stagnated water. This was first noticed by Dr. Priestley, in the 4th volume of his Obfervations on Air; and in confequence of his discoveries, it has been a doctrine generally received, that in common water, and particularly in spring water, expofed to the fun, a green fubftance is formed, which is a copious fource of vital or dephlogisticated air. The Doctor, however, entertained the idea, that this fubftance belonged neither to the vegetable nor to the animal kingdom, but that it was fui generis, a filmy matter not organized, deriving its particular colour from exposure to the fun. This unlucky idea not only prevented the Doctor from making fuch further obfervations as might have terminated in a difcovery, the honour of which was referred for another, but it led him to doubt concerning the truth of his own theory, that vegetables have the power to correct polluted air: for he perceived that vital air continued to be generated in a vessel of water after the plant had been taken out: fuppofing that this filmy fediment neither partook of animal nor of vegetable nature, and perceiving that it was a plentiful fource of vital air, he began to doubt whether the generation of vital air could be afcribed to vegetables; or rather, he decided in the negative. M.INGENHOUSZ obferves, that after having taken infinite pains, for the space of three fucceffive years, to investigate the nature of this fubftance, he was convinced that Dr. Priestley had examined it only when it was in an advanced state. If he had traced its progrefs from its origin, and attentively followed it in all its furprifing changes, which gradually take place, he would probably have entertained very different ideas of its nature; and I have reafon to think, (the author fays,) that the Doctor would not have hefitated to place it in the clafs of beings pertaining to the animal fyftem. Nay, he would have been induced to conclude, that it paffes gradually from the animal to the vegetable kingdom, without totally forfaking all the properties of its primitive nature; and if he had continued his obfervations, as I have done, for feveral years, he would have been still more aftonished to behold this fubftance, after having entered into that clafs of productions which are hitherto fuppofed to belong to the vegetable kingdom, again manifeft indications of animal life.'

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To prove these facts, is the object of the first memoir: twelve fections are devoted to a minute inveftigation of this curious subject, and to establish the doctrine on a firm and folid bafis.

M. INGENHOUSz procured this matter, in the manner pointed out by Dr. Prieftley. Bell-glaffes, filled with spring-water, were inverted on a dish, and expofed to the fun; other veffels were expofed to the fun without being covered. The progrefs of appearances we will relate in a tranflation of his own words:

Some days after exposure to the fun, and after a great number of air bubbles have arifen from the bottom and fides of the veffels, we shall perceive that a greenish cruft is forming. By applying a microscope to the external furface of the veffel, (a compound microscope is preferable, on account of the largenefs of its field,) infinite multitudes of extremely minute and greenish particles will be perceived, of a round or oval form, attached to the internal furface of the veffel, particularly toward the bottom. Sometimes the greatest number will, at the beginning, be found in the upper region. These particles are more clearly viewed, by detaching fome of them with the point of a knife, and placing them in a piece of flat glafs, within the focus of the microfcope. Sometimes I have fufpended flips of glass, by means of threads faftened to a piece of cork, within the veffels; at other times I have placed them at the bottom, in order to view the bodies without difturbing their arrangement. These corpufcles will now difcover themfelves to be indubitably infects, very fimilar to each other, most of them round or oval, or approaching to thefe forms, and envelloped in a glairy and tranfparent film. Some are manifeftly endowed with motion, and pafs with eafe through the water that adheres to the flip of glafs; others are feen to pafs through the whole body of water.. The perfect refemblance between thefe infects, which give unequivocal figns of life, and thofe motion lefs corpufcles that cleave to the fides of the glass, scarcely leave a doubt that they are the fame, and that the latter are rendered motionless, by being glued or entangled in the glairy film. Each of thefe infects, viewed feparately, appears flightly tinged with green but when they are accumulated, their verdure becomes more manifeft. Among the infects adherent to the glafs, a large quantity of hard, tranfparent, angular bodies, with irregular furfaces, are found. They refemble the cryftallization of faline fubftances, or ftoney concretions. These are larger than the infects, and are more or lefs numerous, according to the particular nature of the water that has been employed. These infects multiply perpetually, and attach themfelves, in fucceffion, to the bottom and fides of the veffel; fo that, in the space of a few weeks, the cruft generally becomes of a fine green, particularly toward the bottom of the veffel, feels glutinous to the touch, and is of a confiderable thickness. From every part of this cruft, fmall bubbles of air are feen to arife, as long as the veffel remains expofed to the fun. In the fhade, and during the night, few or none

are formed. The production of this air increases while the green cruft is forming: but afterward it gradually diminishes. If the veffel be fuffered to remain exposed to the heat of the fun, without fresh water being added, the cruft becomes yellow, and afterward of an orange colour, particularly toward the upper furface. If the water be renewed when the production of air is confiderably diminished, it is reproduced in great abundance; and thus by occafionally renewing the water, large quantities of the most pure dephlogifticated air may be obtained from the vessel.

If this cruft be examined at the end of fome weeks, when it has acquired a degree of thickness and confiftence, when it is mucous to the touch, green, and for the most part tranfparent, without being very firm, the microfcope will reprefent it precifely in the state in which Dr. Priestley has defcribed it, in his fourth volume, printed in 1779, p. 342. for it now appears a filmy matter, become green by expofure to the fun; nor are there any traces of organization. Thefe green corpufcles, which were distinctly vifible at the first period of their exiftence, are fo accumulated on each other, and perhaps fo changed in their organization, that the most attentive obferver will fcarcely be able to trace their primitive form, if he has not followed them ftep by step, in all the changes through which they have gradually paffed. If this green cruft be examined fome weeks later, when it has acquired a greater degree of firmnefs, it will be found that the metamorphofis has been in fucceffions. The whole has put on the appearance of a confufed mafs, and of a green gelatinous fubftance dried: but if this be broken, and the edges be examined with a good microfcope, the original green corpuicles will re-appear, enveloped in a glairy matter, which is interlaced with tranfparent fibres, refembling capillary tubes of colourlefs glafs. These fibres poffefs obvious motion; they incline to each other, recede to their former pofition, twift round each other, and difengage themselves again. Thefe movements are repeated at regular intervals. Thefe obfervations require confiderable patience, which will convince the obferver, that the moving fibres are fubftances diftinct from the shining matter; and when viewed at the inflant of their greatest vigour, they appear like animalcula in the form of eels, as lively as thofe which the Abbé Fontana found in the cheese which had the difeafe termed by Linné, granum abbreviatum. They fomewhat refemble, alfo, the eels obfervable in vinegar, excepting that the motion of the latter is tronger: but their fubfequent changes have a closer analogy with thole produced in vegetables. Floating maffes of this green matter are often found on the furface of the water fwollen by the airbubbles confined within them. In thefe, the moving fibres are the most confpicuous. Sometimes a vermicular motion is also apparent, propagated from one extremity of the fibre to the other, refembling the peristaltic motion of fome kinds of worms: but this peristaltic motion requires certain reflections of light to become confpicuous. The filaments retain this motion, and their whitenefs, for a certain time only. If the fubftance mentioned above be obferved at too early a period, no fibres are feen, and at too late a pe

a period, no motion, though the fibres remain confpicuous. It is not easy to determine when the motion will be manifeft: but the more the veffel containing the green matter is exposed to the rays of the fun, the fooner it appears.'

As many of our philofophical readers will naturally be impatient to repeat the experiments on which a doctrine fo novel, and fo interefting to natural hiftory, is founded, we hope that the above detail of the manner in which they were performed, will be acceptable. We were unwilling to omit the minuteft circumftances, as the fuccefs of delicate experiments fo much depends on them.

M. INGENHOUsz further obferves, that this green cruft, in the space of fome months, becomes uneven, feveral irregular tubercles arifing on its furface. If it be ftill left to itself, (the water being fometimes, but not too frequently, renewed,) thefe tubercles become larger, and rife in high and irregular pyramids. At this period, the green fibres which ferpentine irregularly across the inequalities of the green cruft, develope, become more erect, and arrange themfelves conformably to the height of the pyramids. If these pyramidical bodies are clafled among vegetable fubftances, they ought to take their place among the tremella. During the whole courfe of these changes, the fubftance continues to yield dephlogifticated air, when exposed to the fun. The author adds, it continues to yield it while I am writing this, notwithstanding it has already been expofed two years.'

Dr. Prieftley has difcovered, that this green matter is produced much more copioufly, and with the greatest facility, when any animal or vegetable fubftances were added to the water. This is afcribed to the influence of putrefaction, which feems to be the principal cause of this verdure; and therefore it makes its appearance much more flowly in common water, without these additions. M. INGENHOUSZ's obfervations teach him, that the gall of an ox furnithes the largeft quantity of the verdure, among animal fubftances, and indigo among vegetables. When artificial methods are employed, the animalcula are not only more abundant, and the changes made with the greater celerity, but the difference in the infects themselves is the more remarkable; which principally depends on the nature of the putrifying fubftance employed; yet exactly the fame fpecies of infect is not always procured from the fame fubftance. In general, thofe produced by art are larger, and of a deeper green. In fome of thefe, the head feemed feparated from the body, and was only connected with it by means of a very thin filament. The infects from potatoes were the finalleft, and thofe from cow's dung, and pigeon's dung, mixed with the

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