ANNALS

OF

PHILOSOPHY.

INTRODUCTION TO VOL. XIII.

Historical Sketch of the Improvements in the Chemical Sciences during the Year 1818. By Thomas Thomson, M.D. F.R.S.

FROM the commencement of the Annals of Philosophy in the year 1813 to the year 1817, I inserted in the January number an historical sketch of the progress of science during the preceding year. This sketch was confined chiefly to chemistry, and the sciences connected with it. I noticed indeed the different branches of mechanical philosophy, and slightly glanced at some of the departments of natural history. But the primary object of the Annals being chemistry, and my industry being exerted to introduce into them every chemical discovery of importance, which came to my knowledge, in what country soever it had been made, I considered myself as pretty well qualified by the course of reading which the editing of such a journal naturally required to give a tolerably complete view of the progress of chemistry during the preceding year. Hence it naturally happened that these historical sketches often contained many important facts which want of room had prevented me from noticing in the previous numbers of the journal. I flatter myself, therefore, that they would be perused with some advantage by those of my readers who interested themselves in the science of chemistry, and who could not but wish to become acquainted with the various additions which it had just received. My sudden removal to the University of Glasgow, in Oct. 1817, laid me under the necessity of interrupting these historical sketches; and after a trial of two years, I find it difficult to resume them at the usual time; for in the months of October, November, and

December (or at least the last two of them), in which it would be necessary for me to be employed in drawing up the paper, I am almost wholly occupied in teaching, and could not, if I were to make the attempt, spare sufficient time for so laborious a task. We have, therefore, after much consideration, adopted a plan, which bids fair to improve the value of these papers, while it does not interfere with my duties as a professor of chemistryat least so seriously. The plan is to publish two supplementary numbers, each to be prefixed to its respective volume. In the first, or July supplement, we propose to give an historical sketch of the progress of chemistry and mineralogy during the preceding year. This, I trust, I shall be able to draw up myself. The other supplement will contain a sketch of the progress of mechanical philosophy, botany, and zoology, during the preceding year, and will be drawn up by gentlemen well qualified to do justice to their several departments. Such is the plan which will be hereafter followed by the Editor of the Annals of Philosophy, and which it is hoped will meet with the approbation of its readers.

I. CHEMISTRY.

Several very important additions have been made to the science of chemistry during the course of the year 1818. In order to put my readers fully in possession of the facts, I shall be under the necessity of taking up some topics which came under our notice in the historical sketch printed in the Annals of Philosophy for July, 1818; but I shall take care to avoid all useless repetition. The advantages of arrangement are so obvious that I need make no apology for classing the different discoveries under their general heads.

1. LIGHT AND HEAT.

1. Measure of Temperatures.-All the precise notions respecting heat which we have acquired are derived from the use of the thermometer. The importance of this instrument has been long known, and much labour has been bestowed by philosophers in ascertaining the best way of graduating thermometers so as to make them comparable with each other. Now a thermometer is an instrument so contrived as to measure the dilatation of a liquid, and mercury has been found the most convenient liquid for the purpose. When heat is applied to mercury, it increases in bulk, and, rising in a graduated glass tube, indicates the degree of heat to which it is exposed. Several points remain still to be settled before the thermometer, even in its present. improved state, can convey to us precise information. Do equal increments of heat occasion equal increments of bulk in mercury? Or do bodies expand more at high temperatures when they receive an equal increment of heat than at low temperatures? Or at what rate do they expand? These and several similar

questions remain still unresolved. The view which Mr. Dalton has taken of the thermometer and of expansion in the first volume of his System of Chemistry, has drawn the attention of philosophers to the subject, and seems to have led the Academy of Sciences of Paris to make it the subject of a prize, which was gained by MM. Dulong and Petit. A translation of their paper has appeared in our 13th volume. The experiments which it contains seem to have been made with much care; and are, therefore, calculated to decide our opinion respecting this very important but intricate subject. I shall endeavour to lay the facts which these gentlemen have established before my readers.

A preliminary point of some importance was the temperature at which mercury boils; or, in other words, what is the bulk of mercury when heated to the boiling temperature compared with its bulk at the temperature of 32°. Their mode of determining this point was exceedingly ingenious, and appears quite satisfactory. They filled a glass tube, shut at one end, and drawn out into a capillary point at the other, with mercury at the temperature of 32°. The tube thus filled was weighed, and the quantity of mercury which it contained determined. The tube was then kept in boiling mercury till it had acquired the temperature of that liquid; while the pressure exerted by the mercury in the capillary part of the tube prevented the mercury in the tube from boiling or any vapour from being formed. When the mercury was boiling hot, the capillary point of the tube was hermetically sealed by the blow-pipe, and the whole tube was allowed to cool. It was then weighed, and the weight of the mercury which it contained was ascertained. The comparison of this weight with that of the mercury at 32° gave the expansion of mercury at its boiling point; and knowing how much mercury expands between 32° and 212° it was easy to determine at what degree the mercury in a thermometer would stand if the whole of it were raised to the boiling temperature, and if the dilatation of the glass were abstracted. The result of the experiment made in this way is, that mercury boils when raised to the temperature of 360° centigrade, which is equivalent to 680° Fahr.

Another point of considerable importance, and without which indeed the boiling point of mercury could not be determined with precision, was to ascertain the absolute expansion of this liquid at different temperatures. The mode employed, though not altogether new, was, however exceedingly ingenious, and seems to have answered the purpose perfectly. It was founded upon the well-known hydrostatical fact, that if two liquids be poured into the opposite legs of an inverted syphon, the height of each will be inversely as its density. They filled an inverted syphon with mercury. One leg was kept at 32°, by being surrounded with a mixture of snow and water; while the mercury in the other leg was raised to different temperatures, by being surrounded with hot oil. The difference between the height of the

mercury in the two legs was accurately measured at each temperature, and this difference indicated the specific gravity of the mercury in the hot leg of the syphon, or the expansion which it had sustained. The following table shows the dilatation of mercury for a degree centigrade at the various temperatures centigrade, indicated in the first column of the table, and measured by an air thermometer.

From these experiments we learn, that if we employ an air thermometer to measure the temperature, and if we suppose the expansion of air to be equable, or, in other words, that equal increments of heat occasion equal increments of bulk, then mercury is more expanded by heat at high temperatures than at low temperatures, or its expansibility slowly increases as the temperature augments. This rate increases so slowly that between 32° and 212° it sensibly corresponds with the expansion of air; so that we may consider the expansion of mercury as equable up to the temperature of 212°. Between 212° and 392° there is a small increase in the expansibility. There is another small increase between 392° and 582°. The first of these increments, 'as may be seen by the third column of the preceding table, is equivalent to 4.61° of the centigrade scale. The second increment is equivalent to 14-15° of the same scale. The consequence of this is, that 200° centigrade on the air thermometer is the same as 204.61° on the mercurial thermometer, and 300° on the air thermometer the same as 314-15° on the mercurial thermometer.

As we have no other method of measuring temperature but the expansion produced, it is obviously necessary in the first place to fix upon some body as a standard by supposing its expansion to be equable. Our authors have made choice of air. They have taken it for granted that its expansion is equable, and have been induced in consequence to compare with it the expan

sions of all other substances. When we consider that it has been established by experiments which appear satisfactory, that all gases undergo the same change of bulk by the same increments of heat between 32° and 212°; and when we consider further the peculiar constitution of these elastic fluids, it will, I think, be acknowleged, that they are at least as likely, if not more so, to expand equably when heated, as any substances in nature; yet I think it a material defect in the paper of which I am at present giving an acount that a rigid examination of this

point was not undertaken by the authors of it. A little consideration is sufficient to show us that the equal expansibility of the gases cannot be considered as completely established. If I recollect M. Gay-Lussac's experiments (for I have them not at present at hand to consult) they were carried no higher than the temperature of 212°. Mr. Dalton's were also limited by that temperature. Now it is obviously possible, as we see from the experiments contained in the paper before us, that the expansions of the different gases might have agreed with each other up to 212°, and yet have deviated from each other at higher temperatures. Thus the expansion of air and mercury follows the same law up to 212°, but at 392° a deviation is quite perceptible, and at 582° it has become considerable.

There is a method of deciding the question which has been long known to chemists, having been originally tried by Dr. Brook Taylor, and afterwards by Dr. Black, and by Dr. Crawford. I am rather surprised that such active experimenters as Dulong and Petit, who seem to have set out with the resolution of taking nothing for granted, did not have recourse to it. To determine whether equal increments of expansion were occasioned by equal increments of temperature, the philosophers above-mentioned mixed together equal weights of water heated to different temperatures, and observed whether the heat of the mixture was the mean of the temperatures of the two portions of water before mixture. Suppose they had mixed one pound of water at 40° with one pound of water at 100°, and that they found the temperature of the mixed liquid to be 70°, they would have concluded that up to 100° equal increments of expansion were produced by equal increments of temperature. It is well known that Dr. Crawford, from experiments made in this way, concluded that up to 212° mercury expands equably when heated. This conclusion is confirmed by the result of the experiments stated by Dulong and Petit. Now it would have been natural to have had recourse to a similar mode of measuring the expansion of air compared with the temperatures at heats considerably elevated above 212°. Water could not have been used for the purpose; but the fixed oils would have answered sufficiently nearly to the temperature of 600°, and mercury could have been used for still higher temperatures. Indeed a little ingenuity might have enabled them to carry the comparison up to a red heat by means of mixtures of lead or of tin; and thus to have settled a question which must still be considered as a desideratum of material very consequence, because it affects all our measurements of temperature, and all our conclusions respecting heat.

The dilatation of several solid bodies were compared by Dulong and Petit with those of air and mercury. The method was simple and ingenious. Having determined the absolute dilatation of mercury by heat, they measured the dilatation of it in a glass tube. The difference gave them the absolute dilatation of the