These crude processes have been considerably modified in Newfoundland and Norway. The livers reserved for the preparation of medicinal oil are taken as fresh as possible, always within 12 hours of the capture of the fish. They are very carefully examined, and those which are poor or injured, or have pieces of gall adhering, are rejected. The selected livers are then thoroughly washed and dried. They arc immediately put into open barrels, where the oil slowly exudes, and is ladled from the surface. When quite cold, it is filtered several times through blotting-paper, and put up in tin cans or oak barrels. It is of a straw-yellow colour, almost devoid of taste and smell, and is known as "natural medicinal." The livers are then planed in tinned sheet-iron pots, suspended in water contained in a larger pot—a kind of water-bath ; the beating of the water causes the oil to exude from the livers. Sometimes the heating medium is steam, and in other cases, the steam is directly applied to the livers themselves. The temperature at which the operation isieanducted appears to differ most materially. At some works, it is not allowed to exceed 44-io (112° F.); whilo at others, it is raised to 82° (180° F.). The occurrence requiring to be specially guarded against is the breaking-up of the livers before the oil has been removed from the vessel, otherwise infinitesimal portions of animal matter mingle with the oil and subsequently putrefy in it, thus lowering its value. The exuded oil is skimmed off, and " reduced " or " boiled away" till all the water has evaporated from it. It is then filtered four times through filter-paper or very fine muslin, to free it from all remaining solid impurities. The yield of oil depends upon the condition of the fish, which varies exceedingly with the season. Early in the season, they are rich in liver, so that 250-300 fish may give a barrel of liver ; as the season advances, they become poorer, till 600-700 are required to give the same quantity of liver.
The classification of the product naturally depends upon the systems which are in vogue for its preparation. The most usual subdivision is threefold ;—(1) The palest and purest, termed " steam boiled medicinal" or "ordinary bright," used only in medicine ; (2) a somewhat redder after-yield, called " light-brown," inferior for medicinal purposes, but largely used for such ; (3) the " dark brown,' or "tanners'," obtained by roughly boiling down the livers remaining from the foregoing processes. The last is settled in large receiving-tanks, racked off, and barrelled. It is largely used by tanners and curriers. The best is said to be from Newfoundland ; that from Labrador fetches 2-4 cents a gal. less in the market. The refuse solid materials remaining after all the oil has been extracted are added to other offal for making fish guano (see Manures). The chemical and physical characteristics of the three qualities of the officinal oil as classified by de Jongh are as follows :—(1) Palest and clearest : colour, golden-yellow ; sp. gr., 0.923 at 17° (63i° F.) ; soluble in 40 parts
cold and 22-30 parts boiling absolute alcohol ; deposits a white fat at —13° (9° F.) ; (2) pale brown : colour, that of Malaga wine : 6. gr., ; soluble in 31-36 parts cold and 13 parts boiling absolute alcohol ; (3) brown : colour, dark-brown, greenish by transmitted light ; sp. gr., 0.929 at 17° (63° F.); soluble in 17-20 parts cold or hot absolute alcohol ; deposits no solid fat at —13° (9° F). The oil consists chiefly of oleine and margarine, and contains small proportions of iodine, bromine, and free phosphorus, besides peculiar constituents. It is very largely prescribed in medicine, its efficacy being probably due to the bromine, iodine, and phosphorus present, though opinions are not uniform on this point. Many other oils are substituted for true cod-liver-oil. That obtained from the ling (Gadus [Lota] Molva) is recognized by the London pharmacopoeia. The liver-oils of the done (G. cellarius), and the coal-fish (G. carbonarius) were formerly supplied to Great Britain from Bergen, and are still chiefly used in Germany and Scandinavia. The burbot (Lola vulgaris) also contributes to the liver-oil prepared in the Shetlands, &c. Besides these, the oils extracted from the livers of the haddock, hake, eat-fish, conger-eel (p. 1375), ray (p. 1376), shark (p. 1370), and probably many others, are surreptitiously mingled with the cod-liver-oil of commerce.
The trade in cod-liver-oil and its substitutes has attained considerable dimensions, and is of great importance to the population of certain districts. The annual production of Newfoundland is said to amount to 1 million gal., value 200,000/. ; the exports of unrefined cod-liver-oil were 2275 tuna (of 210 gal.), value 113,757/., iu 1864, and 2946 tuns, value 49,618/., in 1878 ; the highest figure reached during that time was 4140 tuns, 138,000/., in 1869 ; the lowest was 2268 tuns, 75,600/., in 1876 ; the exports of refined cod-liver-oil were 171 tuns, 26,3801., in 1864, and 63 tuns, 2520/., in 1878 ; the highest figure reached during that time was was 419 tuns, 27,9441., in 1865 ; the lowest was in 1878. The French cod-fisheries on the Newfoundland coast were said to have yielded an average of 560,000 kilo. of oil in the five years ending 1871. The Norwegian fisheries exported 130,600 barrels, value 386,6001., in 1877 ; the estimated exports in 1878 were 66,000 barrels (of 100 kilo.), of which, 4000 were white steam-prepared oil, 12,000 yellow medicinal, 12,000 common oil for industrial purposes, 8000 brownish-yellow, and 30,000 brown tanning, considerable quantities of which are still used medicinally on the Continent. The Loffoden fisheries, in 1879, afforded about 2750 barrels of medicinal oil, and 33,500 barrels of blubber ; the Finmarken fisheries, in the same year, produced 3080 barrels of medicinal oil, and 25,000 barrels of coarser oil ; the Sfindm6re fisheries also gave some 2700 barrels of medicinal oil. The total exports of cod-liver-oil from Sweden and Norway in 1879 were 143,165 hectol. (of 22 gal.).