The performance of this valve is shown by the table to have been so far satisfactory that the greatest range of pressure between the highest and lowest observations, at any position of the weight, is .6 of an atmosphere. While, therefore, no accurate investigation of the elasticity of steam could result from the use of such a valve, it is found to answer, so far as these results can show, the requirements of practice. In regard to the ratios of the average observed pressures to the calculated pressures, they range from .98 to 1.11, and at a mean the ratio is 1 to 1.034, a ratio which would produce, at higher pressures, marked differences between the calculated and observed pressures.

In a paper by M. Garnier, in the Annales des Mines, vol. 8, the results of his comparisons of safety-valves, having a considerable projection over their valve seat, with others having a small projection, are given, and are very wor thy of attention. He found that a valve, the disk of which projected upon the valve seat beyond the opening of the seat .4 of an inch, opened at a pressure within the boiler, of only two-thirds of that shown by a mercury gauge, and that the distance to which the weight upon the lever of the valve required to be placed from the fulcrum, in another case, for a pressure of four atmospheres, was that corresponding to five atmospheres, if the pressure of the air on the upper part of the valve were estimated. With a disk projecting but .02 of an inch the ratio of the calculated pressure to the observed was 1.06 to 1.

The experiments above quoted, with a disk projecting .1 of an inch, give a less ratio than is indicated by M. Garnier, and show besides that the phenomena are not constant, the individual experiments varying more from each other than the mean varies from the calculated result. The fact developed by M. Garnier is attributed by him to a want of perfect contact between the disk and its seat, and the variations observed by the committee go to strengthen this opinion. Different positions of the disk, different states of its surface, and the interposition of small particles of dirt will satisfactorily account for the want of uniformity in the pressure at which the valve opened when all circumstances were apparently the same. Further evidence on this head will be gathered from the results furnished by the second safety-valve. This it will be recollected, was of the same form and dimensions with the first, and similarly graduated. In the following table the observed temperatures and pressures are given, at which the valve, circumstanced as stated in the column headed remarks, opened. The mean pressure, the highest and lowest pressures, and range, are given in the next four columns. The calculated pressure, and the ratio of the mean pressure to the calculated, are in the next two columns.

Second series.

Ratio of calculated

to observed.

The weights used in the experiments were verified after their conclusion, and found to be accurate. The ratios of the calculated pressure to the observed increase, and afterwards decrease, showing that the results are not attributable to inaccuracy in the unit of weight which was successively applied. They are not explicable by a defect in measuring the valve seat, as is shown by the nearer coincidences at the lower pressures, when the ratios are much nearer to unity than in the higher.

This valve was fitted, by grinding, twice during the experiments, but that the contact with the seat was not perfect, is shown by the number of times at which leaking is noticed before the rise of the valve; these leaks at the high pressures became very sensible, and increased rapidly, from the instant of their first appearing, to that at which the valve opened.

The mean ratio of the calculated to the observed pressure is 1. 10 to 1, but taking for comparison with the first valve, the same range of pressures as was calculated for the first, the ratio is as 1.04 to 1, differing but little from the ratio obtained for the first valve.

This defect in the safety-valve, which affects its use as a means of judging of pressure in proving boilers, is increased as the pressure increases, and may cause an inadequate proof by the forcing pump, or the hydraulic press. No adhesion of an undue kind is shown, by these experiments, to have taken place with the second valve, the difference of pressure of opening and of undue leakage, under similar circumstances, being moderate. The experiments, it should be remembered, apply only to disk valves, and not to those of a conical form.

XI. To ascertain, by direct experiment, the effect of deposits in boilers. The committee have been fortunate on this head in obtaining the account of the results, on a large scale, furnished by the deposits in the boilers of boats on our western waters; to these they refer, as showing,

1. That deposits, consisting of sedimentary matter, carbonate of lime, and other salts, collect in particular parts of boilers, and, preventing the communication of heat to the water, are baked hard, becoming "as hard as brick," when the water is low.*

2. That these collections of mud, &c. may, by causing the undue heating of the bottom of a boiler, produce exfoliations of oxide, which gradually reduce the thickness of the metal; or they may allow the temperature of the metal to be raised so high, that it swells out by the ordinary pressure of the steam, and finally bursts. Thus leading, gradually or suddenly, to the weakening the boiler, and to the discharge of its contents.

The committee have also examined cases of similar deposits in iron boilers, using spring (hard) water. They consisted chiefly of the carbonates of lime and iron, mixed with oxide of iron, containing, besides, the earthy salts from the water. Unless removed at short intervals, they form in the ordinary use of the boilers, and without undue heating, exceedingly hard crusts upon the bottom of the boilers, requiring the aid of the chisel for their removal. Retarding the passage of heat into the boiler, they lead to a great waste of fuel; and exposing the bottom to be unduly heated, they destroy the boiler gradually, by wear, if not suddenly, by explosion.

The nature of these deposits, and the rapidity of deposition, varies, of course, with the kind of water which is used.

[TO BE CONTINUED.]

Explosion of boiler of steamboat Caledonia, W. Littlefield, p. 310, vol. viii., Jour. Frank. Inst. Anonymous, p. 310, ib. Matthew Robison, p. 311, ib. E. W. Benton, p. 314, ib. Also, L. Hebert; p. 379, ib. Thos. W. Bakewell, p. 386, ib. Thos. J. Halderman, p. 28, vol. ix.

† Col. S. H. Long, pp. 244–5, vol. viii. Prof. Johnson, at monthly meeting of Institute.

VOL. XVII.-No. 4.-APRIL, 1836.

23*

FOR THE JOURNAL OF THE FRANKLIN INSTITUTE.

On Calcareous Cements. By JAMES FROST, Civil Engineer.

No. III.*

The affinities of lime for the following substances have been classed by chemists, and seem to have been neglected by them as of little importance, or interest; this has probably arisen from their feeble action at low temperatures, and from chemists not being aware of the value of the investigation. As the temperature increases, the affinity of lime increases for all the substances named, becoming at length very intense. Under atmospheric pressure only, at a bright red, or nearly white heat, in the dark, water and carbonic acid separate from lime without decomposition; under increased pressure, they remain in combination, the carbonate of lime forming into a solid, resembling marble. As the temperature increases, the other substances combine with the lime, and undergo partial decomposition, losing oxygen, which is taken by the lime, and retained by it at common temperatures. This fact, and some other curious laws, have been hitherto unregarded, yet will be found of great importance to the right understanding of the subject we have in hand.

The affinities of lime are regular and constant, as must be expected by all who understand chemistry; they are in the following order.

When we examine the lowest of these affinities, that of water, we shall find it of surprising energy; yet, great as it is, and greater as the preceding ones must necessarily be, as they all displace it, these immense forces are singularly limited by some curious laws, which I shall hereafter endeavour to develope, and also to show, in detail, the peculiar action of magnesia, soda, potassa, hydrogen, sulphuretted hydrogen, carbon, &c., which, in many cases, exerting peculiar and important influences, will, of necessity, require considerable detail. Indeed, when it is considered how many elements are included in these combinations, the necessity of detail will be apparent, to render them easily understood, and extensively useful.

When pure lime is moistened with pure water, twenty-eight parts of lime enter into chemical combination with nine parts of water, and form what is termed hydrate of lime. Any superfluous water may be readily expelled from the hydrate, by exposing it to a heat a little above that of boiling wa ter; but the nine parts of water remain fixed at a red heat, and require a heat white in the dark for their expulsion. When the substances are pure, and in sufficient quantity, they become instantly red hot on admixture; a fact well known by the frequent accidental firing of combustible matter in this way. That this combination is permanent at a red heat, may be easily proved by trial, and that it requires a more intense heat for their separation, is evident from the following experiment. 100 grains of hydrate were enclosed in a small platina crucible, to which a cover was luted, having only

* No. 2 was published in the December number of this Journal, vol. xvi., p. 377.

a small tubular aperture in the centre, and being placed in a reverberatory furnace, at a white heat, became equally hot in five seconds; after the lapse of a minute, a cold iron bar was held, for a few seconds, over the aperture of the crucible, and on withdrawing the bar, a drop of condensed water was appended to it; at intervals of a minute, other bars exhibited the same appearance for nearly a quarter of an hour.

700°

The great heat developed in slacking lime, has been hitherto attributed to the latent heat of the water being set at liberty on its assuming a solid form in the hydrate. Now, the heat that ice absorbs in becoming fluid, is 140°, but the red heat of the water in the hydrate is at least and as the lime is also heated to the same degree, and as the specific heat of lime, compared to that of water, is as .2199 to 1, if we multiply the latent heat of the former by 33, we shall find the heat of the lime,

467

Deduct the latent heat of water,

1167

140

1027

Remaining unaccounted for,

Being more than seven times the latent heat of water, and which we must leave unaccounted for at present.*

That water is not decomposed in combining with lime, is proved by heating one hundred grains of hydrate in a platina retort, connected with an air-tight condenser, expelling the water from the hydrate, and, when cool, returning the same water on the same lime, several times in succession; no extrication of hydrogen, or sign of decomposition, will occur. water held by lime at a red heat, would, if not so held, form steam of immense elasticity, it follows that the cohesive force of lime and water is a still greater force. This expansive force of red hot steam has been differently stated by Rumford, and by Perkins; the lowest statement of the former is 50,000 atmospheres to the superficial inch, more than twelve times the cohesive force of bar iron. What a wonderful exhibition of the power of chemical attraction!

The same compound is formed when lime is exposed for a length of time to the air, from which it certainly, though slowly, absorbs the vapour that may be present in it, and, as we shall hereafter see, with the same proportionate extrication of heat, although, being so slowly extricated, it has hitherto escaped observation by our senses, or by our instruments. Lime, thus air-slaked, as it is termed, also absorbs carbonic acid, and is thus injured, in some degree, for good mortars, and this fact is pretty well appreciated. But its absorption of oxygen seems to have been little regarded, although it is the great source of its use for medical purposes; it is a fact, however, that lime absorbs in slacking, and subsequently as a hydrate, a considerable quantity of oxygen, and hence its use in disinfecting; for, by absorbing oxygen, it liberates nitrogen, which, combining with putrid miasmata, renders it innoxious, or neutral, and, if carefully used, is probably as useful an article as the more expensive, and less well known substances, used for that purpose; but all these absorptions, however useful for particular purposes, are injurious, where good mortar is the object required. These absorptions of gas are visible by slacking some lime in a cup floatOur correspondent here assumes that water, in combining with lime to form a solid, is in the same condition as when frozen; a conclusion which seems ur.tenable.

COм. PUB,