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scattered regularly, fome larger star will attract the neighbouring smaller ones, and their united power will contribute to increase the number ; so that distinct nebulæ will be formed, each of which will comprehend a great number of stars. The nebulæ will be varied in form, from the size of the stars, their original vicinity, and other circumstances; and, when these become numerous, vacant spaces will consequently be left, This theoretical view is very consonant to what really appears; and the agreement is so striking, that it must be very neār the truth. In this theory, Mr. Herschel confiders a projectile force as originally impressed 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 see afterwards reason to think, that this sometimes happens. If we suppose the distance of a star of the first magnitude to be unity, one of the second as at twice the distance, and so on to the seventh, the smallest star that we can, even with common assistance, 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 see to the end of it: for, since, according to the above estimations, he can only extend his view to about seven times the distance of Sirius, it cannot be expected that his eyes should reach the borders of a cluster which has perhaps not less than fifty stars in depth every where around him. The whole universe, therefore, to him will be comprised in a set of constellations, 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 light, 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 situations, let him be placed in a much extended ftratum, or branching cluster of millions of stars, such as may fall under the third form of nebulæ. Here also the heaa vens will not only be richly scattered over with brilliant constellations, but a shining zone or milky way will be perceived to surround the whole sphere of the heavens, owing to the combined light of those stars which are too small, that is too re. mote to be seen. Our observer's fight will be fo confined, that he will imagine this fingle collection of stars, of which he does not even perceive the thousandth part, to be the whole con tents of the heavens.'
All this very nearly agrees with actual obfervations: the different kinds of nebulæ are observed in the forms here theo. retically pointed out; and, instead of an imaginary defcription, it seems actually copied from nature. The nebula which
we inhabit appears, from the results of the table of ftar.gages, to be of the third kind, a very extensive branching compound congeries of many millions of stars. The fection of the heavens, delineated in the plate, is drawn on a scale, in which the di. stance of Sirius is of an inch ; and, in this fcale, all the stars which we see in the finest nights may be comprehended within a circle of less than ; of an inch radius.
It is impossible to follow the very wonderful phænomena which Mr. Herschel points out. He then proceeds to enumerate the different nebulæ, and to consider their distance. In this respect, there must be necessarily much doubt. Some of these seem not to be at less than six or eight thousand times the distance of Sirius. All these circumstances, though apparently so astonishing, rest on a better foundation than many of the theories implicitly believed by mankind.
The paper concludes with an account of some nebulæ, which which are called planetary. The edges are well defined; but they preserve their brightness, with very high magnifying powers, they are not bright enough for single ftars, and too much fo for comets in their aphelion. Mr. Herschel seems to fufpect, that they may have arisen from nebulæ, which, in consequence of various changes, may have rushed 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 easy Method of reducing them to a common Standard. By Richard Kirwan, F. R. S.This article cannot be abridged, because its utility consists in tables, which we cannot transcribe. The title explains its import; and it is executed with great accuracy and ingenuity.
Art. XIV. Electrical Experiments made in Order to ascer. tain 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 so. Mr. Morgan's ingenuity and knowledge of nature would enable him to pursue this subject with advantage.
Art. XV. Experiments and Observations relating to Air and Water. By the Rey.. Joseph Priekley, LL.D. F. R. S.-Dr. Priestley deserves as much praise for candour, as for his abilities. The experiments, before us are a valuable supplement to those of M. Lavoisier ; though there seems to have been a
little difference in the memoir transmitted 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 disappeared, and the inflammable air loft was exactly the quantity sufficient 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 vessel in which the experiment was made. The remaining air was inflammable. The experiment succeeded also in Dr. Priestley's hands with copper and mercury, though M. Lavoisier's experiment with the tube of copper failed. We must, therefore, wait for farther information on this subject.
In other experiments, Dr. Priestley found that water was essential to the production of inflammable air. He was not aware that iron and charcoal, when intensely hot, had so strong an attraction for water, that they would attract it in the midst of the hottest fire, and through the pores of a retort.' He repeated his experiments, and saw that both substances only produced this air in consequence of water being acci. dentally present.
Though Dr. Priestley agrees with M. Lavoisier in the result of his experiments, he differs in the conclusion drawn from them. He still contends for the existence of phlogiston in inflammable air ; though with both English and French chemists he seems to allow, that water is produced by the union of these kinds of air. In his experiments for this purpose, which were made with steam transmitted through charcoal, the inflammable air was united with fixed air, and the latter was often so accurately combined with the former, thatsit appeared only after decomposition. · It seemed probable too, from the quantities expended, compared with the air procured, that it came rather from the other materials than the water. It was somewhat better eftablished by the experiments with iron. We Thall add Dr. Priestley's theory of the changes produced by these experiments, on the supposition that phlogiston really exifts.
• Since iron gains the same addition of weight by melting in dephlogisticated air, and also by the addition of water when red-hot, and becomes, in all respects, the same substance, it is evident, that this air or water, as existing in the iron, is the very same thing; and this can hardly be explained but upon the supposition that water consists of two kinds of air, viz. in.
fammable and dephlogisticated. I shall endeavour to explain these processes in the following manner.
• When iron is melted in dephlogifticated air, we may fuppose that, though part of its phlogiston escapes, to enter into che composition of the small quantity of fixed air which is then procured, yet enough remains to form water with the addition of dephlogisticated air which it has imbibed, so that this cal of iron conGsts of the intimate union of the pure earth of iron and of water ; and, therefore, when the same calx, thus fatusated with water, is exposed to heat in inflammable air, this air enters into it, destroys 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 seem to support the English theory, in opposition to that of M. Lavoisier ; but we cannot particularly relate them. Dr. Priestley concludes this curious paper with a hint that some important discoveries are nearly within our reach.' May this prophecy soon be realized !
The Now Dispenfatory. By William Lewis, M. B. F.R.S. The
Fifth Edition, carefully revised and corrected. 8vo.
Nourfe. The Edinburgh New Dispensatory, containing Elements of Phar.
macy, &c. the two latter Parts comprehending the Preparations and Compositions of the last London and Edinburgh Pharmacopeias ; being an Attempt to collect and apply the later Discoveries to the Dispensatory, published by Dr. Lewis. By Gentlemen of the Faculty at Edinburgh. 8vo. 75. 6d. Elliot,
Edinburgh, Robinsons, London, THE "HE appearance of a new edition of Dr. Lewis's Difpen
fatory, at the same 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 present business only to examine the improvements : indeed the media cal world has already decided on the merits of the former edi. tions; and Dr. Lewis has gained, by his work, a very exten. five reputation.
In the interval between the appearance of the second 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 ; several foreign Dispensatories 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 fame attention as chemistry; nor has it been examined with the indulgent fondness which has distinguished the other branches of the science; 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 instances, have enlarged our knowledge of their respective menftrua. Many new articles of the materia medica are in. deed already almost forgotten, and some cannot yet be procured; but, in a fyftem of this kind, it is necessary that they thould be mentioned, and the little information which we can obtain should be communicated. From the foreign Dispensatories much knowledge can be procured. The best of these, the Wurtenburg Pharmacopæia of 1771, whose authority is very extensive through Germany; the Ruffian, the new Brunf. wick, and the late Swedish Dispensatories, might have contri. buted ufeful aflittance to the present plan. We are sorry to observe that our editors have overlooked, or not been able to procure them. The omission perhaps is rather to be lamented than cenfured.
The conduct of these different improvers, in the latter parts of the work, is nearly the same. The alterations in the last, are those only of the Edinburgh Difpenfatory; and the former professes to corred the formulæ' from the same work. But, while the promises of the first seem to have been pun&tually, 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 these works diverge from each other in this direction), the additions to each are numerous; and the new articles are nearly the same ; 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 interspersed : tables of mercurial and antimonial preparations are subjoined, as well as some articles which are not found in the other. These are the lactuca', one fpecies of which, the virosa, has been recommended in drophies; slichen islandicus,' . nux vomica,' &c. On the contrary, neither the 'quercus marina' (fea wrack), or the viola trico lor,' which have shared the attention of the London editor, ae mentioned ; and, though the citle of the falix is found, 6