TABLE XXIII.

Experiments on copper bar No. 8. Manufactured by John M' Kim, Jr., & Sons, of Baltimore, from South American pig, melted, refined, and rolled into boiler plate, full inch thick. Cut off with the shears

TABLE XXIII.

1inch wide, filed to the dimensions recorded, marked and gauged at every inch. Specific gravity 8.9285.

REMARKS.

Effect of increased temperature on copper.

The effect of temperature on tenacity, has been hitherto but slightly examined, either for theoretical or practical purposes. The general truth that heat diminishes, and eventually overcomes cohesion, is too well established by daily observation to admit of question.

The temperature of no tenacity, is generally supposed to be that at which the fusing point of the given substance is placed, and the point of maximum tenacity ought, upon general principles, to be found at the point where least heat prevails, that is, at the natural zero, or point of absolute cold, if such a point exist in nature. Between these two extremes, it might be supposed that the tenacities of different substances, particularly such as are capable of passing immediately from the solid to the liquid state, would be found to obey certain laws. As the total cohesion at the maximum would present to a mechanical agent tending to overcome it, the whole of its resistance, and as, at more elevated temperatures, a part of that tenacity would be overcome by heat, and the rest must be destroyed by the mechanical force, it is evidently a question of experiment, to decide what relation the two forces have to each other at the several temperatures between the two extremes to which we have just alluded. To decide the theoretical question, or, in other words, to deduce, from the experiments, a law which might be expressed in an abstract form corresponding to all the possible phenomena, would require a state of the materials different from that usually found in commerce or employed in the arts. It would also, as we have seen, require a knowledge of that, about which philosophers no less than practical men, are far from being agreed ;-namely, the point of absolute cold. As the purposes of this committee did not lead them to investigate the problem in all its possible bearings, but only in view of the limits which practice assigns, and with the conditions commonly given to the materials, it will not perhaps, be easy, to construct from the tables a formula in all respects unexceptionable.

The general course of experiments involved the necessity of operating, at the different temperatures, on different bars of copper, and as all the bars are not found to give, even at ordinary temperatures, the same strength, for equal areas of section, it became necessary to deduce from experiments on each bar, at some assumed low temperature, a standard tenacity with which to compare its strength at every other point. The part of this standard tenacity which was taken away by the heat at the higher temperatures, becoming known by the experiment, a comparison was furnished for deciding approximately the relation between the temperature given, and the portion of tenacity which it had overcome. It will be found on an inspection of Table XXIV. containing the comparison of these experiments, that on the eight different bars, the whole number of trials which furnished standards of comparison, at ordinary temperatures, was sixty-six, and consequently on an average about eight trials to each bar; while at the elevated temperatures there were made thirty-nine different experiments at nineteen different points on the scale, the greater number of points, however, having but one experiment each.

An inspection of plate IX., where these experiments are represented, will show that at nearly all parts of the scale, within which the trials were made, the strength diminishes more rapidly than the temperature increases, but some of the higher experiments indicate that the conditions of the law are such as to be represented by a curve, having a point of inflection. It will

also be noticed that the three experiments which appear anomalous, and which in the plate are marked with queries, are all found in trials of the same bar of copper, (No. 7,) and that all these might be referred to a curve, varying but little in form from that which we have traced. It is not however necessary to suppose that these experiments belong to a different curve, for upon recurring to the table of bar No. 7, (Table XXII,) it will be found. that one of the anomalies is satisfactorily accounted for by a delay in taking the temperature after the fracture had occurred, and that one of the others and probably both, were cases of weakening by a slight alloying of the copper by the melted metal through which it passed, in consequence of not having been defended by oxide. The other bars tried at high temperatures were treated with dilute nitric acid, creating a thin film of oxide, which effectually defended the surface, without sensibly diminishing even the smoothness of the bar.

It will be observed that the difference of tenacity, at the lower temperatures, for a difference of from 60 to 90 degrees, is scarcely greater than the actual irregularities of structure in the metal at common temperatures, and consequently, it was not practicable from these experiments alone to deduce a law which should express the tenacities at all points between the maximum above referred to, and the melting point of the metal. Nor would much confidence probably have been reposed in results thus obtained.

In laying down the results in plate IX, the line a b is made to represent the total tenacity of copper at 32°. The horizontal dotted lines express the observed temperatures above 32°, and the vertical ones, the diminutions of tenacity at the respective points.

[TO BE CONTINUED.]

On Calcareous Cements. By JAMES FROST, Civil Engineer.

No. 5.

· [Continued from p. 19, of Volume XVIII.]

By some unfortunate accident the following Essay was lost in transmission by the mail, and its non-appearance induced the author to suppose that we had declined its insertion, as not thinking it of sufficient importance; so far, however, from this being the fact, we unhesitatingly say that we not only view the subject itself as one which is surpassed in importance by very few others, but that we are fully assured that the author of these essays adds to competent theoretical knowledge, an extent of practical acquaintance with the manufacture and employment of hydraulic cements but rarely attained; and he may rest assured that any apparent neglect, should it again occur, ought to be attributed to accidental causes, or, to any thing rather than to a low estimate of the real worth of these EDITOR.

essays.

In my last essay I stated that whatever substances were found in uncalcined limestone, or carbonate of lime, could not be considered as in a state of chemical combination with the lime. This fact appears to be proved by the lime therein being always found to contain its full equivalent of carbonic acid; and any magnesia, also, which may be found therein, is in like manner saturated with carbonic acid, and is, therefore, not supposed to be in chemical combination with the lime. But it is a fact, worthy of remark, that although the carbonates of lime, and of magnesia, are each VOL. XIX.-No 3.-MARCH, 1837.

17

rapidly dissolved, in their separate state, by muriatic acid, the magnesian limestones are acted upon by it with comparative slowness and feebleness. They are, as a class, so distinguished by mineralogists; and this seems to indicate a secondary state of affinities existing between lime and other substances, hitherto wholly unnoticed, and which, in connexion with some other circumstances will be hereafter adverted to in explaining some curious facts.

The other substances most commonly found in limestone, are silex, alumine, and the protoxides of iron and magnesia. The silex as thus found is not only insoluble in muriatic acid, but renders the alumine and metallic oxides partially so, a fact which is also worthy of special observation, because alumine alone is very soluble, and the protoxides, although slowly, are also soluble; nor does the alumine seem to be in chemical combination with the silex, for it is always in the state of an hydrate in clays, and its quantity therein may be estimated by drying the clay for some time in a heat of 212° of Fahrenheit, weighing it, and then heating it to redness for an hour, and reweighing it; as it will then have parted with its water, the weight lost will indicate the quantity, as at 212° the water and alumine in the hydrate appear to be about equal in weight. Alumine is never found in the form of a carbonate, although the reverse of this has been stated in some works. When any one or more of the aforenamed substances are found in sufficient quantity in a state of minute division, and of intimate mixture, either in natural limestones, or in artificial compounds, and subjected to calcination, the calcined mass may prove to be quicklime, slacking with greater or less facility, or a cement incapable of slacking, according to the degree of calcination employed; this is peculiarly the case with compounds of lime and silex. This is a fact of the utmost importance, and yet it is manifest that it is unknown to some persons of great eminence who have treated on cements.

In the Annales de Chimie et de Physique, there is a detailed investigation of the acquired properties of calcined mixtures of gelatinous silex and lime, by that eminent chemist Berthier. From mixtures of two, of three, and of four parts of gelatinous silex, with ten parts of chalk, submitted to calcination, he obtained lime only; yet it will be seen that a cement may be produced from one part only of pulverized silex, with ten parts of chalk, if sufficiently calcined. I communicated this fact, personally, to M. Berthier, in Paris, about seven years since; and about five years ago he published in the Annales, the analysis of a specimen which I gave to him, stating that the cement was used in London, but not giving the source of his informa tion, which I wish he had done, as he scarcely appeared to credit my statement at the time I made it. In his first paper also, he stated that iron was not only useless, but absolutely injurious in cements, whilst in England iron is considered as an essential ingredient in the formation of these substances. We, however, shall find that a cement may be formed by the combination of any two, or more, of the aforenamed substances, but that they each require a peculiar management, and are possessed of very different properties; a point which it is of great importance to understand, as there are some which appear at first the most promising, that are very perishable, whilst others, of much less promise at first, seein destined to be of almost infinite duration.

When the calcination has been sufficient a remarkable change may be perceived in the chemical habits of the combined substances,--the lime no longer slacks, it combines with a smaller quantity of water, or of carbonic