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page 239. The practical use of the gauge-point for gallons will be exhibited more particularly in the short article on Cask Gauging, in the Appendix. On the line A, as laid down above M D, there is generally inserted a pin with the letters IMB (Imperial Bushels) at the point corresponding to the number 2218.192. At 277.274 there is another pin, with the letters I M G (Imperial Gallons). The object of thus distinguishing these particular points is merely to facilitate the setting of the rule when there is occasion to reduce cubic inches into bushels or gallops, or, what is the same, to divide any number by 2218.192, or 277.274, respectively.

Example. Reduce 57500 cubic inches to bushels and gallons.

Set 1 on B to I M B on A; then under 57500, the given number of cubic inches on A will be found 25.9, the number of bushels on B. Again, let 1 on B to I M G on A, and under 57500 on A will be found 207, the number of gallons on B.

It is evident that the operation is one of simple division, as exemplified at page 236. When 1 on B is set to I MB, it is also in a line with 10 on MD; and when set to I M G it is in a line with 8 on M D, as might be expected from the fact that a gallon is the eighth part of a bushel.

Certain scales marked “X, 1st Var.," "X, 2nd Var.," &o, as laid down on some of the modern slide-rules, are connected with theoretical modes of finding the contents of differently shaped casks, and need not be explained in the present work. The line marked “Spheroid," on one of the edges of the rule, will be noticed in the article on Casks, in the Appendix. On the opposite edge there is a scale which shows the number of gross floor bushels to which any number of couchbushels may increase before a « floor-charge" occurs. The construction and use of this scale will be evident on inspection.

CHAPTER V.

Specific Gravity-Hydrometer and Saccharometer-Strength of Spirits and Wort.

(In the original plan of this work it was intended to give a brief article under the foregoing head; but as the Tract on the Manufacture of Spirits, appended to the Distillery Instructions, contains all that is useful or requisite to be known on the subjects, it appears superfluous to enlarge the limits of the present publication by an insertion of similar matter. *

It only remains here to draw attention to Tables 14, 15, and 16, in the Appen. dix, by means of which, as explained in the notes at the foot of each table, the strength of a sample of spirits, either in terms of the per-centage above or below the standard of revenue proof or of absolute alcohol by measure or weight, may be found, together with the amount of concentration on admixture with water, when the specific gravity of the spirit at any one temperature is given.)

* See also the Handbook for Officers of Excise for a minute account of the theory and application of the Hydrometer and Saccharometer, &c.

CHAPTER VI.

CHEMISTRY,

AS APPLIED TO REVENUE PURPOSES. The Microscope-Malting-Malt-roasting-Beer-brewing-Manufacture of Spirits

Rectification of Spirits Methylated Spirit Original Gravities - SugarAdulteration of Articles subject to Ercise Inspection, viz.: Beer, Tobacco, Snuff, Pepper, Coffee, Chicory, Tea, Cocoa.

It is essential to the efficiency of officers of tho Excise Surreying Department, that they should possess, at least, a general knowledge of the chemical changes which take place in malting, brewing, and distilling; and that they should qualify themselves to detect by some simple, readily applied method, the adulterations which are most commonly practised by the manufacturers of and dealers in articles subject to excise inspection. It is also desirable that those officers who have not been instructed at the Board's laboratory should understand the theory of the process for determining the original gravity of beer or distiller's wash, and that they should become acquainted as far as possible with the precise nature and objects of the operations conducted by rectifiers, maltroasters, and other licensed traders.

Although Chemistry as a science or an art, cannot be properly acquired without a course of regular instruction under competent teachers, and the aid of apparatus for experimental purposes, still, there is nothing to prevent any person of average intelligence and industry who may not command these advantages, from learning its first principles, and bringing into use so much of its agency as will enable him to protect the revenue against the grosser forms of fraud.*

The design of the present chapter is to assist the studies of non-chemical officers, by describing in a plain and popular manner the products on which an excise duty is levied, as well as certain articles subject to a duty of customs and placed under the supervision of the excise; the process of forming such products from the raw materials, and the best means of discovering subsequent deteriorations of quality in both classes of substances, without having recourse to refined methods of research, or the use of costly instruments which are of no avail in the hands of any but well-trained observers.

(1.) THE MICROSCOPE. The Microscope is a contrivance, which enables us to sec with distinctness objects placed at a very short distance from the eye, and which also by its magnifying power enables us to discern and examine minute objects that would other. wise be invisible. This instrument is of paramount utility in the examination of substances subject to excise supervision, such as tobacco, snuff, pepper, coffse, &c.,

• One of the best manuals of Experimental Chemistry for the use of beginners, is that tran. plated from the German of Dr. J. A. Stöckhardt, and published by U. G. Bohn, London. Price 52

and in the detection of their more commonly occurring vegetable adulterants. It is not intended to give, in the present place, a detailed account of the construction and various applications of the microscope, as these are fully described in all popular works on the subject, but simply to explain the general principle of its action, and the manner in which it may be most efficiently used for the protection of the Revenue.

The magnifying power of a microscope depends on the property possessed by glasses of a certain curvature, termed lenses, in bending or refracting the rays of light which traverse them in proceeding from an object to the eye. When a ray of light passes obliquely from a rarer into a denser medium, as for example from air into water or glass, it is bent or refracted from its path towards a perpendicular let fall on the surface of the denser body; and when the ray issues into the air again, it is bent from that perpendicular. This will be better understood by reference to the diagram, (Plate 1. Fig. 1.) Thus the ray A O passing into the water B, will not go on to D, but will be bent to E, near the perpeudicular F G, and on leaving the water, instead of going to H, will be turned from the perpendicular F G to I. If the ray penetrate glass or diamond, the amount of refraction will be much greater.

Lenses are either convex, that is, bulging out in the centre, or concave, (bollowed out), or they may consist of a combination of both forms. They may be made of any transparent substance that admits of being shaped and polished. The cheapest, and therefore, the most common material is glass, but diamond is the best, as having the greatest refractive power. Its high price, however, and the labour of cutting and polishing it, are against its general use. The simplest form of a lens is a drop of water, which has the power of refracting the rays of light, and thus, of magnifying to a certain extent any object viewed through it. (Plate 1.) Fig. 2, a represents a plano-convex lens, that is, a lens plane or flat on the one side, and protuberant on the other ; b, a double-convex, protuberant on both sides ; c, a plano-concave, plane on one side and hollow on the other ; aud d, a double-concave, hollow on both sides.

Parallel rays of light falling either on a plano-convex or double-convex lens, are concentrated to a point, called the focus, whilst rays incident on a plano. concave or a double-concave lens, are scattered or made to diverge to a greater or less extent. Thus, in Figs. 3, 4, 5, and 6, the four different kinds of lenses are represented by the letters r, t; parallel rays m, falling on the plano-convex, (3), and double convex (4) are brought to a focus at s; whilst those falling on the planoconcave (6) and double-concave (6) are made to diverge to o, in accordance with the law of refraction stated above. It will be observed that the focus of the rays which pass through the double convex lens, is one half nearer to the lens, than the focus of those which pass through the plano-convex, the former being at the centre of the circle, and the latter at the extremity of its diameter. This difference of focal distance is caused by the inequality in the refractive powers of conrex and plano-consex lenses, the former being exactly double the latter.

Concave lenses do not in any way magnify an object, but are used to correct certain defects, or to make certain modifications in the action of convex lenscs. For instance, rays of light, in passing through the edge or thinnest part of a convex lens are brought to a focus at a point nearer the lens, than those passing through the centre. This is shown in Fig. 7, where the rays n, n, passing through the thinnest part of the lens r, t, are brought to a focus or cross each other at o, and

go on to P, whereas the rays m, m, m, which pass through the centre or thickest part have their focus at s. This defect, which would cause an indistinctness over the object examined is called spherical aberration, and is corrected by cementing to the convex lens, a concave one of particular curvature. Again, a ray of white light, when made to pass through a prism-another form of lens-is found to consist of seven distinct colours, viz., violet, indigo, blue, green, yellow, orange and red. The degree of refraction which each suffers, under the same circumstances, Taries, violet being the most refrangible and red the least, and accordingly the focus of the latter is at a point further from the prism, than that of the former, the foci of the other coloured rays being intermediate between these two extremes. This is called chromatic aberration, and is corrected by combining one or more convex lenses of plate or crown glass, with a double concave of fint glass.

Microscopes are of two kinds, simple and compound. A simple microscope may consist of a single lens or of two or more lenses placed together, either to increase the magnifying power, or to diminish as far as possible the spherical and chromatic aberrations. The simplest in construction is a single plano-convex lens. This is almost devoid of aberration, and when used, the flat side of the lens should be turned toward the object examined. Next in simplicity, but first in usefulness, is the common magnifying glass or pocket lens, an instrument which ought to be possessed and regularly used by every officer in the service. It is so small, that it can be carried in the waistcoat pocket, and yet so powerful, that the majority of excise commodities may be examined with it, and a very correct idea formed as to their genuineness (see articles on Malt, pepper, &c. It generally consists of two, or three, double convex glasses, each having a different magnifying power, and is so constructed, that the three may be used either separately or combined. Fig. 8. (Plate 1.) is a drawing of the pocket lens open ; a, b and c, are the three powers or glasses made to turn upon the hinge d; e is the case in which the three lenses are kept, when not in use, or when brought together under the diaphragm or stop, f. The use of the stop, is to correct as much as possible spherical aberration, by shutting off a portion of the rays of light. For this purpose, the glasses are simply folded into the case, as in fig. 9, and the object viewed through f. The moderate price of a good pocket-lens (from three to four shillings), brings it within the reach of all, and in using it, no difficulty can be experienced, as the object to be examined a grain of malt or barley, for instance-has merely to be held in front of it, at about the distance of one inch, and viewed in various positions.

Another very useful magnifier, and one which possesses certain advantages over the pocket-lens, is that kuown by the name of the “Coddington lens." Fig. 10. (Plate 1.) It is constructed of two plano-convex glasses, cemented together, with a diaphragm or stop between them, to shut out a portion of the light, the whole forming a kind of sphere. Its superiority consists in this : that the rays of light, being compelled as it were, to pass only through the centre of the lens, are nearly all brought to a focus at the same point on the other side of the lens, and consequently the spherical and chromatic aberrations, are, to a great extent, destroyed. Any or all of tho above simple microscopes, may of course, be fixed on stands to suit the convenience of the operator. A compound microscope must consist of two lenses at the least, one called the object-glass, or objective, and the other the eye-piece. Even when two lenses are used in a simple microscope, there is only one image or the object formed, as both are made condncive to the bringing of divergent rays to

a parallel direction. In the compound microscope, on the other hand, two images are produced, the first by the object glass, and the second by the eye-piece. By means of the former, the rays of light are concentrated, and an image of the object is thrown into the focus of the latter, where it is treated as an original object, and magnified still further. In most compound microscopes the object glass consists of a system of from one to eight lenses ; its value depends on its freedom from spherical and chromatic aberrations, and the shortness of the distance at which it brings parallel rays issuing from an object to a focus. The most useful objectglasses are those known as the «inch," « half-inch,” and “quarter-inch." .

The eye-piece is generally constructed of two plano-convex lenses, having their convex sides towards the object-glass. One of these, termed the « eye-glass,” is fixed near the top, and the other an inch or two below it. The latter is called the « field-glass," as it causes the diverging rays from the object, to be brought in a converging direction to the "eye-glass."

An indispensable adjunct to every microscope is a mirror for reflecting light on the object examined, as the latter when highly magnified, requires to have as much light shed on it as if its enlarged size were its real size. The mirror chiefly used is plane on the one side, and concave on the other. The plane side is of great service, in illuminating the margin or boundary of cpaque bodies, whilst the concave side, which is used in examining transparent substances, brings the parallel rays of light to a focus on any required part of the object. It is placed under the stage of the microscope and is so adjusted that it can be turned in any direction.

By referring to fig. 11, all the essential principles of the compound microscope will be readily understood ; a is an achromatic object glass, corsisting of three pairs of lenses, so adjusted as to correct their respective errors; b, the field-glass, and c, the eye-glass, the two latter being included in the eye-piece, the space from a to b representing the body of the microscope. If an object e, suppose an arrow, be placed before the object-glass a, rays of light will be found to proceed, in three different directions, from three different parts of the lens, viz., the top, the middle, and the bottom. Each set of these rays, if allowed to flow on uninterruptedly would come to a focus at the three points f, g, and h, respectively, forming an inverted magnified image, f, h, of the arrow. Through the intervention of the field-glass, b, however, the rays are refracted, and come to foci at the points k, l, and m, forming the magnified image k, m, which now being in front of the eye. glass, is magnified a second time by this lens, as if it were an original object. The image f, h, would otherwise be very indistinct, whereas k, m, is sharp and well-defined ; n, n, is a diaphragm placed in the eye-piece, between the eye-glass and field-glass, to contract as much as possible the rays of light, and make the object viewed more distinct. The rays of light from the time they leave the object-glass, till they enter the eye at d, may be readily traced in the diagram.*

The manner of preparing, mourting and preserving objects or specimens for the microscope, the difficulties which beset all beginners, and the precautions necessary to be observed, are to be fonnd in all works on the microscope, the greater number of which are so moderate in price as to be witbin the reach of all. In the following pages, officers will find a description of the various substances, subject to excise supervision, their most common adulterants, and the methods to be employed for their detection, both by the pocket-lens, and the compound microscope.

* For a price list and description of cheap and usosul microscopes, sec the advertisement at the end of this work.

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