Martin and Grafton's, for a Spirit Black. 75

mable at a high temperature, which renders it more troublesome to use in the process here contemplated, and also would cause the apparatus to require frequent cleaning from the carbonized pitch deposited. In order, therefore, to get rid of the mineral pitch or asphaltum, forty gallons of the tar are to be introduced into a still, as before; and, instead of stopping the operation as soon as the spirit begins to come over, the distillation is continued with a strong heat, so as to force over the whole of the oil and spirit, leaving the residuum of asphaltum in the still: this process, however, is known to every chemist, and need not be further explained.

In Plate VI. is exhibited, at fig. 1. a rude representation of the apparatus employed in preparing and collecting the fine light spirit black, produced by the combustion of the oil and spirit of coal-tar after its having been purified as above described. a, is the brickwork which supports a number of burners issuing from a tube, b, within, and here shewn by dots, as passing along its whole length. Fig. 2 is a section of the brickwork, with the tube, burner, and receiver, as will be described hereafter. The tube may be called the tar main, as it is intended to be filled with tar: it is constructed of castiron, and from it issues several (in this figure twentyfour) jets or burners, c, c, c: any other number may be employed. d, is a furnace under the tar main, the flue of which extends along, for the purpose of heating the tar to the boiling point, in order to facilitate the process. From the main, b, the tar flows into the jets, c; wicks are introduced into the jets, and, when set fire to by a red hot stick, will burn and emit a very considerable quantity of smoke; which it is the object of this apparatus to conduct through many passages, for the purpose of collecting its sooty particles.

There are a number of hoods, e, e, e, or bonnets, as

they are termed, all of which, through their pipes, have communication with, or lead into, a main chimney, f, f, Into these hoods or bonnets, the smoke of the burners ascends, and from thence passes into the main chimney, f, and thence through the smoke tubes into the box, g: here the heaviest particles of the black deposit themselves; but, as the smoke passes on through the farther pipes, a deposition of the second, or finer, particles of black takes place in the box, h. From hence the smoke proceeds through other pipes into a series of canvas bags, i, i, i, which are proposed to be about eighteen feet long, and three feet diameter. These bags are connected together at top and bottom alternately, and through the whole series the smoke passes up one bag and down the next, depositing the fine black, called spirit black, upon the sides of the canvas. After the jets have continued burning for several days, the bags are to be beaten with a stick, so that the black may fall to the bottom; and, when a sufficient quantity has accumulated, the bags may be emptied and swept out. Thus, seventy or eighty bags may be employed; so that the smoke should pass through a length of about four hundred yards, the farthest of which will be found to contain the finest black. The last bag should be left open, in order to allow the vapour to escape into the open air.

The main tar tube will require to be emptied every four or five days, in order to clear it from the pitchy matter that may have subsided from the burners, and they also will require to be frequently poked with a wire, to clear off the black which forms upon their edges, and to drive down the carbonized tar which attaches itself to the upper part of the jets.

Inrolled, December, 1821.

Original Communications.

On the Reduction of any Volume of Gas at one Temperature, to its Volume at another.

As it is important, in experimental chemistry, particularly where subsequent comparison is required, to obtain every possible accuracy in the reduction of any volume or quantity of gas at one temperature, to its volume or quantity at another, I have constructed a table for this purpose on different principles to those usually met with.

Without entering largely on the subject, I shall proceed to describe the mode by which the table subjoined is calculated, leaving it afterwards to the judgment of others to discover the errors of the customary method, as applied to the same purpose.

If the assertion be true that all gaseous fluids have the property of maintaining a regular rate of expansion when submitted to heat, there is no other mode by which it can be uniform than by the increment of any individual degree being a proportional part of the volume at that degree of heat next preceding, and of no other. It will therefore be a series in geometrical progression; where, calling the volume V, the ratio R, and placing T for one degree of temperature, the following terms are produced: V, VRT, VR2, VR3, VR4T, &c. &c. When only in possession of the first and last terms, and the number of degrees of temperature, and being required to ascertain all the intermediate terms, it is necessary to ᏙᎡᏎᎢ

find the ratio; therefore, &c. = R1, the root of which power being extracted, the number sought for is procured.

The method here represented has been the one attended

to in forming the new table; and, having assumed, on the authority of M. Gay Lussac, that 1.0000, &c. measure of gas, at 32° Fah. becomes 1.375 at 212° Fah. the common

1

ratio of increase is found to be nearly, or more cor565'

rectly in multiplying decimals 1·0017704. Now, if unity be fixed at 60° Fah. as a common standard to which the temperature of the gases are to be reduced, all the terms for the temperatures above this number will increase progressively in the proportion just named; and the respective results when obtained, being transposed, or placed opposite to degrees of heat equally distant in number below 60°, as the former are above, they become multipliers for any definite quantity of gaseous matter of the temperature which is there expressed.

1

But if the ratio by which any term increases be if

565'

the first term be 1, the second is 1 +

; it is

1

or

therefore manifest that the ratio of decrease is

566'

as before, decimally, 1.0017669. Therefore, multiplying a quantity of any temperature by this number, gives the decrement for the next preceding degree. Thus, all the respective volumes belonging to each temperature having been ascertained, supposing as before unity at 60°, they are placed in the table equidistant above this number, and are multipliers for the temperatures opposite to which they stand, giving volumes at 60°. This completes the table, which is confined to a range of from 20° Fah. to 100° Fah.

There is little occasion to remark that a table computed after this manner will give a different result to one

1

made upon the supposition of the ratio being 480 though,

On the Reduction of the Volumes of Gas. 79

even allowing this to be correct, no notice is taken of the progressive increments or decrements, nor of the ratio in which the terms should decrease.

Being employed upon the subject with which I have just concluded, it occurred to me that it might also be serviceable to construct a table of the expansion of mercury by heat, more especially for the purpose of correct, ing barometrical observations. It is true that the variation of the barometer from temperature, as applicable to meteorology, is perhaps unimportant; but, in estimating the heights of mountains, &c. whose summits are usually cold, a range of 10° Fah. with the barometer at 30 inch, would occasion an error of 37 or more feet.

I have accordingly annexed a table, the first column of which determines the simple expansion of mercury. The second is the corrected expansion of the same, supposing the scale of the barometer to be formed in its whole length of cast brass, and to be attached, or to coincide, at the point zero only.

As a general observation, relating to whatever error may arise from the elongation of the scale, it will be irregular when fixed either at the upper part of the barometer, centrally, in its whole length, or if but partially metallic; but, when fixed as before described at Zero, the metal of which it is composed, dilating in the same direction as the mercury, its gradations enlarge uniformly from the same point, and may therefore be allowed for with greater facility.

Though perfection in any instrument where delicacy is required is perhaps impracticable; and though errors arising from the dimensions of the scale varying by heat may be very minute, yet it is desirable to approach as near to truth as the difficulties with which we contend will admit. The improbability of procuring a scale divided with accuracy, may be urged as an objection to notice