This behaviour is apparently due to the presence of the lime, the explanation being that the fatty acids always present in the oils and varnishes used for the paint combine with the lime to form compounds which, from the standpoint of the chemist, must be regarded as soaps. The small lumps already mentioned really consist of lime soap, and the formation of these colourless compounds accounts for the lessened covering power. Given a fine white clay, otherwise capable of forming a valuable pigment, it is sometimes possible, by simple means, to eliminate accompanying lime, provided the amount of the latter is not too great, and also provided that very cheap hydrochloric or acetic acid is available. The acid need not be pure, and the impure but very strong pyroligneous acid, which is very cheap on account of its empyreumatic smell, may be used. To eliminate lime from the clay, the still moist levigated mass is introduced, in small quantities, into a vat containing the requisite quantity (see later) of hydrochloric or acetic acid, the addition being continued until the liquid gives only a faintly acid reaction with blue litmus paper. When the clay is run in, effervescence is produced by the liberation of the carbon dioxide displaced by the stronger acid employed. The amount of lime present in a clay may be determined by very simple means. A small sample of the clay is dried by artificial heat, until of constant weight, and exactly 100 parts by weight of the dry mass are placed in a glass and suffused with hydrochloric acid, sufficient of the latter being used to make the liquid still strongly acid after effervescence has ceased. The contents of the glass are transferred to a filter, and washed with pure water so long as the washings continue to redden blue litmus paper. The residue is then dried until of constant weight, and the difference between the initial and final weights will give the percentage of substances soluble in hydrochloric acid. After performing this simple test on a clay, it is easy to calculate the quantity of acid needed to extract all the soluble constituents from a given weight of the material. All that is necessary is to measure the volume of acid required to extract a small quantity of the clay completely. Thus, if one pint of the acid at disposal is sufficient to treat one pound of the clay, the amount needed for a given quantity of clay is a simple matter of calculation. Since, on account of the cost of pure hydrochloric acid, crude acid will always be used, it will be necessary to remember that this crude acid always contains ferric oxide in solution--this being the cause of its yellow colour. If the amount of acid taken is barely sufficient to combine the whole of the lime, leaving the latter slightly in excess, the ferric oxide--which would otherwise tinge the clay yellow--will be precipitated.