FERMENTATION 351 



of tho boer are observed. Opposite to the spirit-indication of the beer in the table, 

 wo find the corresponding degrees of gravity lost, which last, added to the extract- 

 gravity of the beer, gives its original gravity. 



'Suppose the sugar beer exhibited an extract-gravity of 7'9 (1007'9), and spirit- 

 indication of 11. The latter marks, according to the table, 477 of gravity lost, 

 which added to the observed extract-gravity, 7'9, gives 55'6 of original gravity for 

 the beer (1055-6).' 



Similar tables are constructed for starch-sugar, and for various worts with and 

 without hops. 



After explaining many points connected with the problem, as it presented itself 

 under varied conditions as it respected the original worts, the Keport proceeds : 



' The object is still to obtain the spirit-indication of the beer. The specific gravity 

 of the beer is first observed by means of the hydrometer or weighing bottle. The 

 extract-gravity of the beer is next observed as in the former method ; but the beer 

 for this purpose may be boiled in an open glass flask till the spirits are gone, as the 

 new process does not require the spirits to be collected. The spiritless liquid remain- 

 ing is then made up to the original volume of the beer as before. By losing its spirits, 

 the beer of course always increases in gravity, and the more so the richer in alcohol 

 the beer has been. The difference between the two gravities is the new spirit-indi- 

 cation, and is obtained by subtracting the beer-gravity from the extract-gravity, 

 which last is always the higher number. 



' The data in a particular beer were as follow : 



Extract-gravity . .'" i- . . 10447 

 Beer-gravity . -. .';... . 1035'! 



Spirit-indication . ,, .-*. . 9'6 



' Now the same beer gave by distillation, or the former method, a spirit-indication 

 of 9 '9. The new spirit-indication by evaporation is, therefore, less by 0'3 than 

 the old indication by distillation. The means were obtained of comparing the two 

 indications given by the same fermented wort or beer in several hundred cases, by 

 adopting the practice of boiling the beer in a retort, instead of an open flask or 

 basin, and collecting the alcohol at the same time. The evaporation uniformly indi- 

 cated a quantity of spirits in the beer nearly the same as v is obtained by distillation, 

 but always sensibly less, as in the preceding instance. These experiments being 

 made upon fermented liquids of known original gravity, the relation could always be 

 observed between the new spirit-indication and the degrees of specific gravity lost by 

 the beer. Tables of the degrees of spirit-indication, with their corresponding degrees 

 of gravity lost, were thus constructed, exactly in the same manner as the tables which 

 precede; and these new tables may be applied in the same way to ascertain the 

 original gravity of any specimen of beer. Having found the degrees of spirit-indi- 

 cation of the beer by evaporation, the corresponding degrees of gravity lost are taken 

 from the table ; and adding these degrees to the extract-gravity of the beer, also ob- 

 served, the original gravity is found. Thus the spirit-indication (by the evaporation 

 method) of the beer lately referred to, was 9'6, which mark 43 of gravity lost in the 

 new tables. Adding these to 1044*7, the extract-gravity of the same beer, 10877 is 

 obtained as the original gravity of the beer.' 



The results of the extensive series of experiments made, were, that the problem 

 could be solved in the two extreme conditions in which they have only to deal with 

 the pure sugars entirely converted into alcohol. 



' The real difliculty is with the intermediate condition, which is also the most fre- 

 quent one, where the solid matter of the beer is partly starch-sugar and partly ex- 

 tractive ; for no accurate chemical means are known of separating these substances, 

 and so determining the quantity of each in the mixture. 



' But a remedy presented itself. The fermentation of the beer was completed by 

 the addition of yeast, and the constituents of the beer were thus reduced to alcohol 

 and extractive only, from which the original gravity, as is seen, can be calculated. 



'For this purpose, a small but known measure of the beer, such as four fluid 

 ounces, was carefully deprived of spirits by distillation, in a glass retort. To the 

 fluid, when cooled, a charge of fresh yeast, amounting to 150 grains, was added, and 

 the mixture kept at 80 for a period of sixteen hours. Care was taken to connect 

 the retort, from the commencement, with a tube condenser, so that the alcoholic 

 vapour which exhaled from the wash during fermentation should not be lost. When 

 the fermentation had entirely ceased, heat was applied to the retort to distil off the 

 alcohol, which was collected in a cooled receiver. About three-fifths of the liquid 

 were Distilled over for this purpose ; and the volume of the distillate was then made 

 up with water to the original volume of the beer. The specific gravity of the last 



