fcattered regularly, fome larger ftar will attract the neighbouring fmaller ones, and their united power will contribute to increase the number; fo that diftinct nebula will be formed, each of which will comprehend a great number of stars. The nebulæ will be varied in form, from the fize of the stars, their original vicinity, and other circumstances; and, when these become numerous, vacant fpaces will confequently be left, This theoretical view is very confonant to what really appears; and the agreement is fo ftriking, that it must be very near the truth. In this theory, Mr. Herschel confiders a projectile force as originally impreffed on these bodies, to avoid the objection, that if this account were true, we know of no power which could impede their falling into each other. We fee afterwards reason to think, that this fometimes happens. If we fuppofe the distance of a star of the firft magnitude to be unity, one of the fecond as at twice the distance, and fo on to the seventh, the fmalleft ftar that we can, even with common affistance, perceive,

-It follows, that an observer, who is inclosed in a globular cluster of stars, and not far from the center, will never be able, with the naked eye, to fee to the end of it: for, fince, according to the above eftimations, he can only extend his view to about feven times the diftance of Sirius, it cannot be expected that his eyes should reach the borders of a cluster which has perhaps not less than fifty ftars in depth every where around him. The whole univerfe, therefore, to him will be comprised in a fet of conftellations, richly ornamented with scattered stars of all fizes. Or if the united brightness of a neighbouring cluster of stars should, in a remarkable clear night, reach his fight, it will put on the appearance of a small, faint, whitish nebulous cloud, not to be perceived without the greatest attention. To pass by other fituations, let him be placed in a much extended ftratum, or branching cluster of millions of ftars, fuch as may fall under the third form of nebulæ. Here alfo the heavens will not only be richly fcattered over with brilliant conftellations, but a fhining zone or milky way will be perceived to furround the whole sphere of the heavens, owing to the combined light of those stars which are too fmall, that is too re mote to be feen. Our obferver's fight will be fo confined, that he will imagine this fingle collection of ftars, of which he does not even perceive the thoufandth part, to be the whole contents of the heavens."

All this very nearly agrees with actual obfervations: the different kinds of nebulæ are observed in the forms here theoretically pointed out; and, instead of an imaginary defeription, it seems actually copied from nature. The nebula which

we

we inhabit appears, from the refults of the table of ftar-gages, to be of the third kind, a very extenfive branching compound congeries of many millions of ftars. The fection of the heavens, delineated in the plate, is drawn on a fcale, in which the diftance of Sirius is of an inch; and, in this fcale, all the ftars which we fee in the finest nights may be comprehended within a circle of less than of an inch radius.

It is impoffible to follow the very wonderful phænomena which Mr. Herschel points out. He then proceeds to enumerate the different nebula, and to confider their distance. In this refpect, there must be neceffarily much doubt. Some of thefe feem not to be at lefs than fix or eight thousand times the distance of Sirius. All these circumftances, though apparently fo aftonishing, reft on a better foundation than many of the theories implicitly believed by mankind.

The paper concludes with an account of fome nebulæ, which which are called planetary. The edges are well defined; but they preferve their brightness, with very high magnifying powers, they are not bright enough for fingle ftars, and too much fo for comets in their aphelion. Mr. Herschel feems to fufpect, that they may have arifen from nebulæ, which, in confequence of various changes, may have rusheḍ together, and been destroyed; but he purposes to examine them more particularly.

Art. XIII. Remarks on Specific Gravities taken at different Degrees of Heat, and an eafy Method of reducing them to a common Standard. By Richard Kirwan, F. R. S.-This article cannot be abridged, becaufe its utility consists in tables, which we cannot tranfcribe. The title explains its import; and it is executed with great accuracy and ingenuity.

Art. XIV. Electrical Experiments made in Order to afcertain the non-conducting Power of a perfect Vacuum, &c. By Mr. William Morgan-The non-conducting power of a perfect vacuum is in this article well ascertained, by the author's having produced one more perfect than we have yet known. When the air is rarefied it becomes a conductor, though, in its common state it is no longer fo. Mr. Morgan's ingenuity and knowledge of nature would enable him to pursue this fubject with advantage.

Art. XV. Experiments and Obfervations relating to Air and Water. By the Rev.. Jofeph Priestley, LL. D. F. R. S.-Dr. Priestley deferves as much praife for candour, as for his abi-, lities. The experiments, before us are a valuable fupplement to those of M. Lavoisier; though there feems to have been a

little difference in the memoir tranfmitted to Dr. Priestley. from that which was published.

Iron melted in pure air, by a burning lens, was found to absorb the air almost entirely; the remainder was fixed air, and the iron was reduced to a calx; it increased in weight in proportion to the air absorbed. On reducing it, the increased weight difappeared, and the inflammable air loft was exactly the quantity fufficient to faturate the pure air which had originally disappeared; that is, it was enough to have completely exploded together. The additional weight which the iron had loft, however, appeared in the form of water, that covered the fides of the veffel in which the experiment was made. The remaining air was inflammable. The experiment fucceeded alfo in Dr. Priestley's hands with copper and mercury, though M. Lavoifier's experiment with the tube of copper failed. We muft, therefore, wait for farther information on this fubject.

In other experiments, Dr. Priestley found that water was effential to the production of inflammable air. He was not aware that iron and charcoal, when intenfely hot, had so strong an attraction for water, that they would attract it in the midst of the hotteft fire, and through the pores of a retort He repeated his experiments, and faw that both substances only produced this air in confequence of water being accidentally prefent.

Though Dr. Pricfley agrees with M. Lavoifier in the refult of his experiments, he differs in the conclufion drawn from them. He ftill contends for the existence of phlogifton in inflammable air; though with both English and French chemifts he seems to allow, that water is produced by the union of these kinds of air. In his experiments for this purpofe, which were made with fteam tranfmitted through charcoal, the inflammable air was united with fixed air, and the latter was often fo accurately combined with the former, that it appeared only after decompofition. It feemed probable too, from the quantities expended, compared with the air procured, that it came rather from the other materials than the water. It was fomewhat better established by the experiments with iron. We shall add Dr. Priestley's theory of the changes produced by thefe experiments, on the fuppofition that phlogifton really exifts.

• Since iron gains the fame addition of weight by melting in dephlogisticated air, and alfo by the addition of water when red-hot, and becomes, in all refpects, the fame fubstance, it is evident, that this air or water, as exifting in the iron, is the very fame thing; and this can hardly be explained but upon the fuppofition that water confifts of two kinds of air, viz. in

flammable

flammable and dephlogisticated. I fhall endeavour to explain these proceffes in the following manner.

• When iron is melted in dephlogisticated air, we may suppose that, though part of its phlogifton efcapes, to enter into the compofition of the fmall quantity of fixed air which is then procured, yet enough remains to form water with the addition of dephlogisticated air which it has imbibed, fo that this calx of iron confifts of the intimate union of the pure earth of iron and of water; and, therefore, when the fame calx, thus faturated with water, is expofed to heat in inflammable air, this air enters into it, deftroys the attraction between the water and she earth, and revives the iron, while the water is expelled in its proper form.'

Some other curious experiments are added, which feem to fupport the English theory, in oppofition to that of M. Lavoifier; but we cannot particularly relate them. Dr. Priestley concludes this curious paper with a hint that fome important difcoveries are nearly within our reach.' May this prophecy foon be realized!

The New Difpenfatory. By William Lewis, M. B. F. R. S. The Fifth Edition, carefully revised and corrected. 8vo. 75. 6d. Nourfe.

The Edinburgh New Difpenfatory, containing Elements of Pharmacy, Sc. the two latter Parts comprehending the Preparations and Compofitions of the laft London and Edinburgh Pharmacopeias; being an Attempt to collect and apply the later Difcoveries to the Difpenfatory, published by Dr. Lewis. By Gentlemen of the Faculty at Edinburgh. 8vo. 75. 6d. Elliot, Edinburgh Robinsons, London.

THE appearance of a new edition of Dr. Lewis's Difpen

fatory, at the fame time with an attempt to improve and amend it, has led us to compare these new publications with that on which they are formed. It would be useless and im pertinent to enlarge on the original plan; for it is our prefent bufinefs only to examine the improvements: indeed the medi cal world has already decided on the merits of the former edi. tions; and Dr. Lewis has gained, by his work, a very exten. Live reputation.

In the interval between the appearance of the fecond edi tion, for that was greatly improved by the author himself, and the present time, chemistry has almost become a new science; many articles have been added to the materia medica; feveral foreign Difpenfatories have appeared with great applause; and the Edinburgh college have published two very improved edi

tions of their own. We may take this opportunity of informing physicians, that there is reason to expect a new one, from the college of this kingdom. It is indeed to be regretted, that pharmacy has not shared the same attention as chemistry; nor has it been examined with the indulgent fondness which has diftinguished the other branches of the fcience; yet we certainly owe to the labours of chemists a more intimate acquaintance with the nature of metals, and a more exact mode of preparing metallic remedies: we more clearly understand the nature of vegetable and animal substances; and, in a few inftances, have enlarged our knowledge of their respective menftrua. Many new articles of the materia medica are indeed already almoft forgotten, and fome cannot yet be procured; but, in a fyftem of this kind, it is neceffary that they fhould be mentioned, and the little information which we can obtain should be communicated.-From the foreign Difpenfatories much knowledge can be procured. The best of these, the Wurtenburg Pharmacopeia of 1771, whofe authority is very extenfive through Germany; the Ruffian, the new Brunf wick, and the late Swedish Difpenfatories, might have contributed ufeful affistance to the prefent plan. We are forry to observe that our editors have overlooked, or not been able to procure them. The omiffion perhaps is rather to be lamented than cenfured.

The conduct of these different improvers, in the latter parts of the work, is nearly the fame. The alterations in the last, are thofe only of the Edinburgh Difpenfatory; and the former profeffes to corre the formula' from the fame work. But, while the promises of the first seem to have been punctually, those of the last have been carelessly, executed. The new formulæ are not added, and few marks of correction occur.

In the Materia Medica, (we choose a retrogade order, since thefe works diverge from each other in this direction), the additions to each are numerous; and the new articles are nearly the fame; the fources from which they are transcribed are also not very different. In the London edition, however, the former remain unchanged; in that of Edinburgh, addi tions are freely interfperfed: tables of mercurial and antimonial preparations are fubjoined, as well as fome articles which are not found in the other. Thefe are the lactuca', one species of which, the virofa, has been recommended in dropfies; lichen iflandicus,' nux vomica,' &c. On the contrary,

neither the quercus marina' (fea wrack), or the viola tricolor,' which have fhared the attention of the London editor, are mentioned; and, though the title of the salix is found,