progress of fermentation, and also upon the finished and matured beers. It is obvious that in data so obtained, the anomalous and irregular effects which are attendant upon vinous fermentation, and which render the deduction of the original gravity by calculation impossible, are duly accounted for and elucidated.

The nature of the data thus sought to be established, and upon which the determination of original gravities must always depend, may now be explained.

When yeast is added to wort, the specific gravity of the liquid diminishes as fermentation proceeds; this effect, which is usually termed the "attenuation of the worts" is produced as follows:-Anhydrous starch sugar becomes converted, under the influence of yeast, into alcohol and carbonic acid, thus :

Or, 180 parts by weight of the sugar furnish 92 parts of alcohol and 88 of carbonic acid, but the last-named substance being gaseous, escapes; therefore, nearly half the weight of the sugar which is decomposed in the formation of alcohol, disappears from the wort, the density of which is consequently lowered by such abstraction of solid matter. Again, the density of a solution of sugar being greater, whilst that of alcohol is considerably less than that of water, it follows that the levity occasioned by the alcohol, in attenuated wort, counterbalances and conceals a portion of the density of the remaining sugar, so that before the true specific gravity due to solid matter in such a wort can be determined, the alcohol present must be removed, and the wort made up to its original bulk by the addition of water. The true specific gravity being thus obtained, it is evident that if we add to it the number of degrees of gravity lost in the wort by the decomposition of the sugar in the production of the particular amount of alcohol which it contains, we shall arrive at the original gravity of such wort. To determine the exact proportion between the amount of alcohol present in a wort or beer, and the degrees of gravity lost in the formation of such alcohol, and to do this through a wide range of quantities so that the data should be applicable to worts of various specific gravities, and at all stages of fermentation, was, therefore, the principal object of the researches of Messrs. Dobson and Phillips.

The data recorded by these gentlemen, and upon which they based the method which is now used by the revenue, were of four kinds :-1st. The specific gravity of the unfermented wort. 2nd. The specific gravity of the fermented wort. 3rd. The specific gravity of the spirit obtained by the distillation of a given bulk of the fermented wort, after such distillate had been increased to an equal bulk by the addition of water. And 4th. The specific gravity of the residue after distillation, also increased by the addition of water to the same bulk as that of the wort subjected to distillation.

Of these four descriptions of data, the last three of which were in each case derived from observations made during the same experiment,-the first corresponds to the "original gravity" of the wort; the second represents the specific gravity of the matured beer or the "beer gravity;" the third shows the alcoholic strength of the beer, 1,000 minus the specific gravity, being termed the "spirit indication;"

and the fourth indicates the "extract gravity," or what the specific gravity of the beer would be if uninfluenced by the presence of alcohol. This last, taken in conjunction with the third, shows the number of degrees of gravity lost in the formation of the alcohol.

The data thus established, subject to a slight but variable correction for the acetic acid always present in beer, are applicable to two different modes of arriving at the original gravity, namely, the "evaporation process," similar in principle to those proposed by Balling and Stevenson, and the more accurate « distillation process," which is the only one that has ever been used in this country for revenue purposes. These processes will be fully described in the sequel.

In the year 1847, Messrs. Dobson and Phillips having completed their research, laid before the Board of Excise a method in detail by which the original gravities of beers could be determined, and which, after its accuracy had been severely tested, was adopted and immediately put in practice by order of the Board. The extent to which the revenue had suffered, and the necessity of some efficient check upon the declarations made by exporters, now became strikingly apparent, it being found that out of a large number of samples of beer examined, no less than 74 per cent. were disentitled to the amount of draw back claimed, some of these proving to be as much as twenty degrees below the declared gravity.

The irregularities thus brought to light must not be ascribed wholly to intentional fraud, but partly to the rough and inaccurate methods practised by brewers in observing the density of their worts, and also to the very careless manner in which beers were made up for exportation. Indeed, this want of precision may be fairly said to have been, in a great degree, induced and encouraged by the total absence at the time, of any fiscal control over the declarations of exporters.

As was to be expected, the application of the newly devised test occasioned much dissatisfaction, and elicited loud complaints on the part of persons concerned in the exportation of beer. It was strongly maintained that the system gave erroneous results and was unjust to the trade. The Board, accordingly, in 1851 referred the subject to Professors Graham, Hofmann and Redwood, with a request that they would submit it to a thorough investigation. These chemists performed a series of elaborate and extensive experiments, not only with a view to test the correctness of the method in question, but also to elucidate and explain the laws generally which govern vinous fermentation. The data obtained by them, and embodied in their valuable report addressed in August, 1852, to the Chairman of the Board of Inland Revenue,* are all but identical with the results previously arrived at by Messrs. Dobson and Phillips; while the inquiry fully proved, that the method devised by the last-named gentlemen is both sound in principle and accurate in detail.

Before proceeding to an account of the «distillation process," as practised by the revenue officers, it will be proper to describe the kind of still now employed. Shortly after that process came into effect, it was found that the ordinary glass retort was quite unsuitable, from the time lost in connecting it perfectly with the condenser in each experiment, the difficulty in transferring from it, without any waste, the residue or extractive matter, and the risk that a portion of the beer in * Report upon "Original Gravities " by Professors Graham, Hofmann, and Redwood, and cor. respondence with Messrs. Thorne & Co. (Brewers,) thereon. A copy of this report is supplied to every chemical officer.

the retort might, from its tendency to swell upon boiling, be carried over mechanically into the receiver.

Mr. Phillips, observing these defects, contrived the following very convenient and efficient distilling apparatus, which is well adapted also for a variety of similar operations on the small scale.

A is a glass distilling flask, capable of being connected with the apparatus by the brass screw cap and flange B; this connexion is perfectly air-tight. C is a copper condenser, having, instead of a worm, a series of shallow rectangular chambers arranged in a zigzag form, and by means of which the vapour is exposed to a very large condensing surface; H, represents the condenser with the front side removed, for the purpose of showing the interior arrangement; D D are the water supply and discharge pipes, the former extending almost to the bottom of the condensing cistern C; E is a small glass flask, holding, when filled up to a mark on the neck, about four fluid oz., this is used both as the measure flask and receiver; F is a moveable bracket for the support of the receiver E; G is a stand with telescope or sliding tube; the gas lamp or other source of heat is applied beneath the flask A, when the latter is charged with the liquid to be distilled, and attached to the arm of the condenser at B. The advantages of this apparatus consist in its great condensing power, the easy and perfect manner in which the distilling flask can be attached to the metal tube, and the facility with which it can be detached and rinsed out.

The "distillation process," as practised in the Inland Revenue Laboratory, is as follows, the whole of the weighings and measurings being executed at the temperature of 60° Fah. :-About half-a-pint of the beer is decanted into a clean, dry glass

vessel, and the free carbonic acid gas present expelled as far as possible by pouring the liquid backwards and forwards several times from one such vessel into another. After this treatment the measure flask is filled up to the mark on its neck with the beer, which is then transferred to the distilling flask, the traces of beer remaining in the former vessel being rinsed out with small quantities, not exceeding a third of the measured bulk, of distilled water, and the rinsings added to the beer in the distilling flask; about two-thirds of the contents of the retort are then carefully distilled off, and received in the clean measure flask; the distillate, which contains the whole of the alcohol of the beer, in the form of a weak, impure spirit or low-wines," is increased by the addition of distilled water to the same bulk as that of the beer operated upon, the liquids well mixed by inverting the flask while the thumb is placed over its mouth, and the specific gravity ascertained by the weighing bottle, 1,000 minus this specific gravity, being the "spirit indication." Whilst the distillation is proceeding, the specific gravity of the beer itself is determined in a similar manner-this « beer gravity" is necessary to be known if it be wished to check the results of the experiment by those afforded by the " evaporation" process.

The residue in the distilling flask, when cooled, is next carefully transferred to the clean measure flask, the former flask well washed out with small quantities of distilled water, and the washings added to the measure flask, which is then filled up to the mark on its neck by the further addition of water; this liquid containing the whole of the solid or extractive matter of the beer is shaken up, and its specific gravity observed by the weighing bottle; its density is termed the "extract gravity."

To ensure an exact adjustment of temperature in the various measurings and weighings, two small thermometers especially adapted for the purpose are employed. One of these is fitted with a narrow scale, so that it may enter the measure flask and be freely moved about in the liquid; in the other, the scale is attached to part only of the length of the mercury tube, so that the naked bulb may reach to the bottom of the specific gravity bottle. When the former instrument is taken out of the liquid, it is carefully washed down into the flask with a few drops of water, to prevent any loss of spirit, &c. A specific gravity bottle of the capacity of 1000 grains of distilled water at 60° Fah. is always used in these experiments, in conjunction with a good, chemical balance, as the indications of an hydrometer or gravimeter would not give the densities with sufficient exactness.

Provided there were no acetic acid in the beer, sufficient data would now have been obtained by which the original gravity could be deduced; but this acid is always present in beers, and being produced by the oxidation, or at the expense of a portion of the alcohol previously formed, it is obvious that the spirit indication derived from the specific gravity of the distillate is less than it would have been if no conversion of alcohol into acetic acid had taken place; it becomes necessary, therefore, to determine the exact quantity of acid present, and, according to the equivalent amount of alcohol, to make an addition to the observed spirit indication, which, when so corrected, will show by the subsequent table (No. 2,) the true number of degrees of gravity lost in the production of both the alcohol and acetic acid.

The following is the mode of ascertaining, and making allowance for, the amount of acetic acid in beer.

A solution of ammonia is prepared of such a strength, that a given bulk of it will exactly neutralise one per cent. of absolute acetic acid in an equal bulk of beer, so that if 100 fluid grains of the solution are sufficient to neutralise the acid in 1,000 fluid grains of beer, such beer contains one-tenth per cent. of acid. Ordinary solution of ammonia diluted with pure water until it has the specific gravity 9986 at 60°, is of the exact strength required.

A glass measure tube, or alkalimeter, graduated downwards to the bulk of 1,000 grains, in 100 equal divisions, is filled to 0 of the scale with the ammonia solution; so much of the solution is then added, a few drops at a time, to 1,000 measured grains of the beer, as is necessary to neutralise the acid present, every division of the tube, corresponding to ten fluid grains so emptied, indicating 01 per cent. of acetic acid in the beer. The progress of the neutralisation is tested from time to time with a slip of reddened litmus paper, which should be suffered to become faintly blue before ceasing to add the ammonia. But the whole of the acid present must not be taken into account in correcting the observed spirit indication, as a certain portion is always formed during the fermentation of the worts, and which, from the experimental nature of the data upon which the method is based, is already allowed for in the table of « degrees of spirit indication with corresponding degrees of gravity lost." The quantity of acid thus formed in the fermentation of beer-worts was carefully estimated by Professors Graham, Hofmann, and Redwood to be one part in one thouand parts of the wort, or one tenth per cent.; it is, therefore, only the excess above this which is taken into account in arriving at the true number of degrees of gravity lost.

The following table, No. 1, shows the addition to be made to the spirit indication derived from the specific gravity of the distillate, for any excess of acetic acid above one-tenth per cent.

TABLE NO. 1.

For correcting the spirit indication of Beer, &c., on account of acetic acid.

After the Spirit Indication has been thus corrected, the corresponding number of degrees of gravity lost by the formation of alcohol and acetic acid in the beer, is shown in the annexed table.