228 MADRAS FISHERIES BULLETIN VOL. XIII, 



his goods at an improved price immediately after'production and 

 thus be ready to take advantage of a favourable market. 



Certain substances when mixed with the oil at once coagulate 

 and precipitate albuminous matters ; such are tannin which forms 

 an insoluble precipitate, but this cannot be used where the oil is 

 intended for edible or medicinal purposes ; milk when added to 

 oil and heated similarly coagulates, especially if the oil is acid, 

 and precipitates impurities. Other substances such as fullers' 

 earth, kieselguhr (diatomite, etc.) though inert or almost so in 

 themselves, act mechanically on the foreign particles. These im- 

 purities, etc., are readily precipitated by settlement in tall re- 

 ceptacles, the clarification being greatly improved as well as 

 hastened by the admixture of the foreign materials (kieselguhr, etc.) 

 which operate mechanically as they fall to the bottom. But in all 

 these cases — and necessarily so in the case of fullers' earth which 

 gives a specific earthy taste to the oil if in contact with it except 

 for a very short period — it is usual to filter the mixture when the 

 coagulated substances or foreign materials are arrested by the 

 filter (and, in the case of materials such as kieselguhr, form the 

 most efficient element in the filter) so that the oil issues perfectly 

 bright, clear, and free from impurities. 



128. Filtration. — For the purest and best forms of oil, especially 

 when intended for edible or medicinal purposes, filtration is 

 essential. The usual method of filtering in large modern factories 

 is that of the filter press which consists of a number of connected 

 chambers arranged vertically in a common frame in which they 

 are kept in close contact ; each chamber is of considerable 

 superficial area compared with its width or capacity. The shallow 

 plates forming— in pairs — the chambers are lined with filter cloths 

 and are connected by a common duct through which the fluid to 

 be filtered is pumped at a pressure which may amount to many 

 pounds to the square inch. The filtrand •" is forced through the 

 filter cloth, depositing within the chamber and on the inner 

 surface of the cloth, the impurities, coagulated matter, or foreign 

 materials ; the filtrate (filtered oil) is at first slightly turbid but as 

 the deposited layer increases the oil becomes perfectly cfear and 



*I use the word " filtrand "— though not apparently in present use- to signify 

 ihe fluid to be filtered ; it includes the whole fluid with its impurities, admixtures, etc , 

 and obviates the necessity for lengthy phrases such as " the impure fluid to be filtered," 

 etc. " Filtrate "— that which has been filtered— is of course in common use. 



