the 'Transactions' of which it was published. It will be sufficient here to give an epitome of its contents. The paper was entitled "On the Solubility of Fluoride of Calcium in Water, and its relation to the occurrence of Fluorine in minerals, and in recent and fossil plants and animals." It is divided into seven sections. The first, entitled "Introductory Remarks," details the researches of my predecessors, including those to which I have just referred. The second, entitled "Of the Solubility of Fluoride of Calcium in Water," points out, that, contrary to previous belief, this salt is dissolved by pure water, yielding a solution answering to all the tests of lime and of hydrofluoric acid. The third, entitled "Of the presence of Fluorine in Well-, River-, and Sea-Water," confirms and extends the observations of previous analysts on the occurrence of a dissolved fluoride in fresh i water, and for the first time announces its direct discovery in sea-water, where Middleton and Dana had independently anticipated its presence, after finding it invariably in the shells of marine mollusca and in corals. The fourth section, entitled "Of the presence of Fluorine in Minerals," does not call for special notice. The fifth, entitled "Of the presence of Fluorine in Plants," confirms the results of Will of Giessen as to the existence of this element in the ashes of vegetables, and draws attention to plants and to water as the media by which fluorides may be transferred from the soil to animals. The sixth section, entitled "Of the presence of Fluorine in Animals," commences with the statement, "As there exists, then, a twofold source of fluorine for animals, we may anticipate its occurrence in various parts of their structure;" and thereafter announces, in opposition to the negative results of Dr. Rees, my confirmation of the observation of Berzelius that a fluoride is present in human urine, a result which the great Swedish chemist hailed with satisfaction before his death*, although M. Nicklès seems to think that he has been the first to confirm the original assertion. The paper then proceeds to state, "It could not be doubted, after the facts I have detailed, that fluorine would be found in the two great formative liquids of the animal body, blood and milk; I have found it in both. So far as I am aware, it has hitherto been overlooked in all the analyses that have been made of these liquids; probably it has not been sought for. I employed the blood of the Ox, and in two cases obtained markings on glass which only became visible when breathed upon, but are then quite manifest. In the third the glass was distinctly though faintly corroded."

The concluding part of this section is occupied with a criticism * Jahres-Bericht, von Jacob Berzelius, 1848, p. 164, which contains a general comment on my researches of 1846.

of the declaration of Treviranus, that the gastric juice of birds contains hydrofluoric acid; the final sentence being, "We may now look for fluorine in all the animal fluids."

I merely name the title of the seventh section, which is headed "Of the presence of Fluorine in Fossil Bones and its relation to Animal Life."

In the summer of the same year, 1846, I ascertained the extent to which pure water dissolves fluor-spar, namely 0.26 grain in 7000 grains of the liquid at 60° F. This result was announced to the British Association at its meeting for that year, and to this Society in November. In 1849 these observations were repeated with certain variations, to meet objections which had been raised to my conclusions, but with the same result. In 1849 I communicated to the British Association the results of a series of analyses, demonstrating by a new method of inquiry the presence of fluorine in the waters of the Frith of Forth, the Frith of Clyde, and the German Ocean; and in March 1850 I communicated to this Society an additional series of observations made in the same way, but extended to the waters of the Irish Sea, of the Atlantic, and the Mediterranean. This paper was accompanied by a letter from Professor Forchammer of Copenhagen, testifying to the presence of fluorine in the waters of the Baltic. In the summer of the same year (1850) I returned to the analysis of blood and milk for fluorine, feeling assured that still more decisive proofs of its presence in both could be obtained by using a larger amount of material, and subjecting it to a simpler process. Accordingly, employing in the case of blood (which was that of the Ox) 26 imperial pints, in the case of milk 9 imperial pints, and in that of cheese 12 lbs., I was able to etch glasses with the hydrofluoric acid evolved from them so deeply that they might have been printed from, like copper plates. The etched glasses were shown to the members of the Chemical and Physiological Sections of the British Association at its meeting in Edinburgh in 1850, and the details of the process published in its Transactions,' as well as in the Edinburgh Philosophical Journal for October of that year*. In the spring of 1852 I again brought the subject before this Society in a paper entitled "On two new Processes for the Detection of Fluorine when accompanied by Silica; and on the presence of Fluorine in Granite, Trap, and other Igneous Rocks, and in the ashes of recent and Fossil Plants." (Read April 19, 1852.) In the summer also of the same year, a communication, founded on an application of these processes, was made to the Botanical Society of Edinburgh, They are specially referred to in the English translation of Lehmann's Physiological Chemistry,' by Prof. G. E. Day, vol. i. p. 425. Cav. Soc. Publ. 1851.

"On the presence of Fluorine in the stems of Gramines, Equisetaceæ, and other Plants, with observations on the sources from which vegetables derive this element." In this communication, read July 8, 1852, I reported the results of an examination of twenty-four plants or vegetable products, in twelve of which fluorine was found. It will suffice to state, in reference to both papers, that their object is to point out and illustrate by examples, methods of readily discovering fluorine in circumstances which previously rendered its detection difficult.

The more perfect of the two processes has been applied with success by Professor Hofmann to the detection of fluorine in the mineral waters of Harrogate; and Fresenius has introduced it into the last edition of his 'Qualitative Analysis*.

The researches thus referred to have been chiefly published in the Transactions' of this Society, and of the British Association, but have been brought in part before the Chemical Society of London. They are known in Germany, Denmark, Sweden and America, and have been referred to by many authors in this country. It is reasonable, accordingly, to infer that some knowledge of them has reached Paris; and it might have been supposed that they had not altogether escaped the notice of M. Nicklès, whose name appears on the title-page of the Journal de Pharmacie et de Chimie, as editing the department of that work entitled "Une revue des Travaux Chimiques publiés à l'Etranger."

I bring no charge, however, against M. Nieklès. In these days of multiplied monographs it would be unjust to blame any man for ignorance of a single series of special researches. Nevertheless, seeing that this author's name appears on the title-page of the Journal de Pharmacie side by side with those of our VicePresident Dr. Christison, as its Edinburgh Correspondent, and of Dr. Redwood, the Secretary of the Cavendish Society, as its London Correspondent, the countrymen of M. Nicklès may think themselves entitled to quote the legal maxim, " de non apparen. tibus et de non existentibus eadem ratio," and to infer that what of reputed English science is not known to him, does not exist to be known. Whilst, therefore, I wish M. Nicklès all success in extending our knowledge of the organismal distribution of fluorine, I ask from him, now that he is made aware of the fact, acknowledgement of my priority in reference to the discovery which he specially claims, and of the other discoveries which the papers referred to announce.

*Fourth edition of the English translation, 1855, p. 134, stated by its editor, Mr. J. L. Bullock, to correspond with the eighth German edition. The process essentially consists in heating the silicated fluoride with oil of vitriol, and condensing the gaseous fluoride of silicon in aqueous ammonia, which after evaporation, re-solution in water, and desiccation, yields fluoride

XXVII. On the Heat in the Sun's Rays. By ELISHA FOOTE*.

THE

HE experiments here detailed were instituted for the purpose of investigating the heat in the sun's rays.

Two instruments have been used for this purpose. One was Leslie's differential thermometer. Both bulbs of it were blackened by holding them in the smoke of burning pitch. When experimenting, one was shaded, the other was exposed to the direct action of the sun's rays; and as both were thus equally subject to all other influences, the result was not affected by them.

Generally, however, I have found it more convenient to use two mercurial thermometers, and note their difference. Two small and very delicate instruments were procured as nearly alike as possible. The stems of both were attached to the same plate, about 2 inches apart, and the scales were marked upon it in juxtaposition, so that the eye could see the indications of both at the same time. Both bulbs were blackened as in the other instrument. It was used in the same manner. The temperatures in the sun and in the shade were noted, and their difference was taken as equivalent to the indications of the differential thermometer.

The question that first arises is, Does the difference between the shaded and exposed bulbs afford a correct measure of the heat in the sun's rays? To this point I would ask attention before proceeding to the experiment.

The theory of the differential thermometer was accurately investigated by Leslie. In one of the foci of two parabolic reflectors he placed a tin canister, which was heated or cooled by putting in liquids of different temperatures, or frigorific mixtures. In the other, the heat was received on one of the bulbs of his differential thermometer: and under all circumstances, the indications of the instrument were found to be accurately proportional to the differences between the temperatures of the canister and those of the surrounding air.

I have varied these experiments by keeping the canister at the uniform heat of boiling water in different temperatures of the air, and by substituting other sources of heat, and have always found the results to accord with those obtained by the distinguished philosopher to whom I have referred.

The principles of radiation lead to the same result; for while of ammonium. Fresenius recommends the addition of "some coarse pieces of marble to ensure a continuous slight evolution of gas;" but I cannot approve of this recommendation, since the constant occurrence of fluorine in shells and corals implies its presence in limestones; and the employment of marble for the purpose indicated, risks the introduction of the very element for which we are seeking.

Read before the American Association for the Advancement of Science, August 23, 1856; and reprinted from Silliman's Journal for November 1856,

the differential thermometer receives heat from the canister, it at the same time radiates it to surrounding bodies, and that in proportion, or nearly so, to the difference between its temperature and that of the medium in which it is placed.

I regard it therefore as well established, that the differential thermometer affords a correct measurement of the differences between the heat of the canister and that of the surrounding air. These differences may evidently be varied in two ways: by changing either

1st. The heat of the canister; or—

2ndly. The temperature of the air.

An increase or diminution in the heat of the canister would directly increase or diminish the differences; whilst an increase in the temperature of the air would diminish the difference until an equality between the two was obtained. If the temperature of the air were uniform, and the changes were those of the canister alone, the instrument measuring the differences would correctly indicate those changes; but if the heat of the canister were uniform and that of the air were varied, then would the instrument equally indicate those changes, but in a contrary direction. In case the heat of both the canister and the air was varied at the same time, if we knew the change in one and its effects upon the instrument, we could easily deduce the changes in the other. Suppose, for example, an increase of ten degrees on the scale of the instrument, and an elevation of five degrees in the temperature of the air; the effect of the latter having been to depress the thermometer five degrees, and the canister having not only overcome that effect but increased the indications ten degrees, the sum of the two or fifteen degrees would be the real change which had taken place in the heat of the canister. Had there been a depression in the temperature of the air, it obviously should be subtracted from the indications of the instrument to obtain the desired measurement.

It is upon these principles that I have applied the differential thermometer to measure the comparative heat in the sun's rays. One of its bulbs received their direct action in the same way that it received the rays proceeding from the canister. The temperature of the air was at the same time obtained by a common thermometer. An increase was added to, and a diminution subtracted from, the indications of the instrument to obtain the real changes in the heat of the rays proceeding from the sun.

My first experiment was of the simplest kind. It was a winter's day. The differential thermometer was placed on the outside of a window where the temperature was below the freezingpoint. The effect measured by the scale (which merely divided the stem into equal parts) was 53°. It was then placed on the inside of the window, where the temperature was about 70°, and