the water. This would afford a far better explanation of their active powers than the composition usually assigned to them. They might still be viewed as very active solutions; they might be considered as equally powerful with the most soluble and the most active salts which they are capable of forming by binary combinations. It is not altogether impossible that their simultaneous combination might even confer additional powers. Probably most chemists will incline to adopt this view of the subject. Dr Murray rejects it, because, if fairly followed out, it would lead to the conclusion that all combinations of compound bodies are simultaneous combinations of the primary elements -aconclusion from which no inference with regard to specific qualities could be drawn, and which would, therefore, be inconsistent with the conclusions which, in many cases, we are able actually to form. It is probable that most other chemists will see less weight in this objection, and will be disposed at least to acknowledge that the exact relations subsisting between the primary ingredients of a complicated compound, whether in a fluid or in a solid state, lie probably for ever beyond the reach of actual determination. This consideration itself prepares us to acknowledge with less mystery or reluctance, the existence of any powers in mineral waters to which experience lends its countenance, and, where the facts are in conformity to the presence of such a state of combination as can be at all assigned to the simple ingredients, we can be at no loss to say that all the powers which such a state implies are explained as the result of the composition; and, in addition to this, we may conceive other accumulated chemical agencies to be at the same time concerned in the operation.

The labours of this chemist in the "Analysis of Sea Water," are too ex

VOL. XI. PART I.

tensive to admit of any abstract in this place. It is sufficient to remark the advantage imparted to the results by the application of the methods of reasoning which he had adopted with regard to mineral waters. He is in this way enabled to reconcile with one another the analyses given by his predecessors, sometimes at variance. For example, he accounts for the singularity which appeared in that of Lavoisier, who obtained from it portions of sulphate of soda and muriate of lime, ingredients found by no one else. Dr Murray, in repeating with exactness the process of Lavoisier, as well as those of other chemists, found that the difference of result depended on the process employed. The alcohol employed by Lavoisier favoured the formation of the crystals which he obtained.

The separation of the different salts by crystallization is tedious and difficult, and seldom perfect in the end; and, as this laborious mode of procedure gives us no information regarding the mode of existence of acids, alkalis, and earths, in a mixed chemical solution, he proposes that we should, in all such cases, satisfy ourselves with determining the acids and salifiable bases and their respective proportions, by means of reagents which have the power of precipitating them. He found in a pint of the sea water which he employed,

Following up these improved views, he lays down, in his third Memoir, "A formula of general application for the analysis of mineral waters." He adheres exclusively to that method which has been usually called the indirect, which consists in discovering the acids and bases, without deciding any thing regarding their mode and order of combination; while the direct method consisted in the obtaining of separate crystallized or precipitated salts, and solutions containing only one salt each. This last had been considered as giving not merely the ingredients, from which an opinion or conjecture might be formed regarding their constitution, but as declaring that constitution in the first instance. The author, however, having shewn that no direct information of this kind is afforded by such analysis, proposes that it should be relinquished as far less satisfactory than the indirect method. In this last, we have it in our power to ascertain the proportions of the constituent parts with much greater accuracy; and having done this, we infer the composition by reasoning on such principles as have been now explained. These principles, if they do not afford absolute certainty, will preserve us from the errors of precipitate deduction which have been hitherto acquiesced in, and enlarge our views of this class of objects. The salts usually found are carbonates, sulphates, and muriates of lime, of magnesia, and of soda. After trying, in a general way, what acids and bases are present, using nitrate of barytes for ascertaining the presence of sulphuric and carbonic acids, and nitrate of silver for muriatic acid; ascertaining the presence or absence of lime by oxalic acid, of magnesia by lime water or ammonia, and of any alkaline neutral salt by evaporation; he directs a series of steps for ascertaining the proportions of the respective prin

ciples.

These do not admit of abridgment; and, therefore, we must satisfy ourselves with a reference to the author's Memoir, not doubting that it will be quickly copied from the Transactions into works more extensively circulated among persons interested both in general and in practical chemistry. In the course of it some acute remarks, in the form of improvements, suggested by the author's practice in manipulation, and substantiated by his own experience, are interspersed, and the whole business of analysis is likely to derive from them a material degree of accuracy, as well as simplification. It is important farther to remark, that they are shewn by the author to admit of an easy extension to the analysis of earthy minerals.

IMPRESSIONS OF COLD TRANSMITTED FROM THE HIGHER ATMOSPHERE, AND THE NEW METEOROLOGICAL INSTRUMENT CALLED THEÆTHRIO

SCOPE.

Some very important experiments were made, a few years ago, with great labour and care by Dr Wells of London, on the temperature of different parts of the surface of the ground, as influenced by the nature of that surface itself. These are detailed in his Essay on Dew, containing one of the most meritorious series of purely experimental investigations that modern times have produced in the department of meteorology, and beautifully elucidating some new and interesting applications of the chemical doctrines of heat. From these it appears, that the same sorts of surface which give out heat most powerfully by radiation, and which receive most readily the heat which is radiated from other bodies, those surfaces also which radiate cold most readily, (all which qualities uniform

ly co-exist in the same proportion, in any surface, shewing that they depend on the same superficial constitution), are also liable to the greatest reduction of temperature when exposed in the night to a clear and dry atmosphere. For this reason, dew and hoarfrost are more copiously deposited on these surfaces than on others. The reduced temperature makes the portions of the atmosphere which come successively in contact with the surface deposit their humidity. It is well known to chemists that in this separation caloric is given out; hence, in some experiments formerly made by Mr Patrick Wilson of Glasgow, which were read to the Royal Society of London in 1788, and others, which are contained in the Transactions of the Royal Society of Edinburgh, vol. 1st, it appeared an inexplicable circumstance that, where dew or hoarfrost had been deposited, the temperature was particularly low. This is now fully explained. The low state of the temperature is prior to the deposition, and the cause of it; and though that deposition raises the temperature in proportion to its own amount, it does not necessarily raise it to that of the air and other surrounding objects. This cold is in itself inde. pendent of the presence of moisture, and the dew produced is in proportion to the reduction of temperature, and the impregnation of the air with moisture. The experiments were made by placing a number of thermometers on the ground; some on gravel, others among grass, and others on smooth stone, or on metals; and it was found that, when the sky was overcast even in a slight degree, all the thermometers stood about the same degree of temperature; but that when the sky was perfectly clear, a great difference took place,-those in contact with the most radiating surface always indicating the lowest temperature. The cold is occasioned by

THE RADIATION OF CALORIC FROM A SURFACE OF THIS KIND TO THE UPPER REGIONS OF THE ATMOSPHERE, or to regions altogether beyond its limits. The upper regions, in fact, operate in the same manner with the bottle of snow which, in the experi ments of Pictet and others, radiates cold on surrounding bodies.

Mr

Professor Leslie has taken up the subject in a more discriminating and accurate manner, reducing the esti mate of such effects to measure and calculation. The results of his inquiries, and a description of the ingenious and beautiful instrument with which he operated, were given in a paper read before the Royal Society of Edinburgh in March last. Leslie's opinions on the radiation of heat have been long before the public, and in this paper they are repeated and illustrated. He considers this class of phenomena as depending on the presence of the air. They do not, according to him, consist in the simple transmission of caloric through space, but in certain appulses among the particles of the air, which proceed on all sides in radiations like sound, or like the rippling waves on the surface of a liquid, which proceed from the disturbed point, producing circles which become wider as the effect of the impression is extended. It is in their propagation through air that these effects are best (and we may say exclusively) known to us. It is not easy to ascertain the reality of the diffusion of such powers through a perfect vacuum; but it is in favour of Mr Leslie's views, that these are propagated more powerfully through a dense than through a very rarefied atmosphere. Mr Leslie objects to the term. radiation. He considers the effects produced as a series of internal oscillations, by which the aerial medium successfully transfers its charges of caloric, and delivers an im

pression at the end of the chain of communication of the same kind precisely as it had received at the beginning. Mr Leslie tried the difference between the surface of the ground and the atmosphere a few inches above it, by means of the differential thermometer. He found, that in sunshine and calm weather the ground was sometimes 30 millesimal degrees warmer than the air only a few inches above it. But when the sky happened to be much overclouded, or when strong winds swept over. the surface, the accumulation of heat hardly reached three degrees. Fresh ploughed land, or a surface spread over with hay, indicated more than twice the effect that appeared on fine pas

ture.

Mr Leslie, in the course of these experiments, found that, towards evening, if the sky was clear, the thermometer on the ground indicated a greater cold than in the atmosphere, (unless it was protected by a polished metal, or a substance which reflected the rays of heat,) although the ground itself was still warmer than the air. This led him to suspect, that an opposite impression was by some means communicated from the atmosphere at these times, and he was induced to investigate this set of influences. For this purpose he introduced, under the sentient ball of his pyroscope, (that is, that ball of the differential thermometer which remained without a metallic covering, while the other had one, and which consequently was most readily operated on by those impressions of temperature proceeding from distant bodies which are called radiations)- under this he introduced a small circular plate of tin hammered into a slight concavity. This more than doubled the action of the instrument, and, therefore, put the existence of these impressions beyond all doubt. The radiations

which reached the concave metallic surface were reflected so as to accumulate the effect on the ball placed in a focal situation. After some varied experiments suggested by this fact, with a view to the more accurate determination of the laws observed by these impressions, as indicated by the variations of their amount under different circumstances, he contrived a set of very ingenious and useful instruments, by means of which some further facts were made known. He exposed a pyroscope in the focus of a paraboloid to the influence of the sky at different times, and to different quarters of the sky at the same time. It was necessary to guard against the disturbing influence of wind. This was first done by putting his pyroscope with the small reflector within a deep pitcher by which the lateral impulses of the wind were intercepted; and afterwards, instead of this arrangement, he made the reflector sufficiently deep to answer that purpose of itself. The form which he adopted was that of a truncated oblong spheroid of metal, cut through the upper focus by a plane perpendicular to the axis, finely polished on its inner surface, so as to reflect the impressions of cold or heat, and having the sentient ball of the pyroscope placed in the lower focus. This instrument indicated most fully the action of that quarter of the heavens to which it was turned. He therefore had an instrument which was mounted on a pivot, so as to be conveniently turned to any portion of the heavens which it was his object to explore. This instrument, when covered with a thin plate of glass, often shewed one or two millesimal degrees of heat, the effect of the radiation of the light of the sky. It was when this screen was removed, and the reflecting surface and sentient ball exposed to the sky, without any intermedium except