The proportion of silica in this is higher than in many clays used for brick- or terra-cotta making, but the alkalis, potash and soda, are in strong force, so that any refractoriness on the part of the silica is soon subdued in the kiln. The iron, also, is in abundance. The principal colouring ingredient is the sesquioxide, and we can quite understand the manufacturer when he informs us that, in spite of the rich tint of the goods produced, nothing is artificially mixed with this clay to produce such a result. We may call attention to the method of expressing the chemical analysis in this case, which might be copied to advantage. In the first place, the combined and the uncombined iron are separately shown, or rather the degree of combination is indicated; and secondly, the proportion of water chemically combined is differentiated from that which has simply soaked into the clay, though expelled, following a well-known practice of chemists, prior to commencing the analysis proper. It is of very little use giving the amount of water, unless the proportions are divided in this manner. In the result given above we learn that there is very little chance of the clay shrinking, as it only contains moisture to the extent of 1.54 per cent.; but if that had been added to the water combined, we should have had a result of 5.08 per cent., which is not nearly so clear in its meaning. We may add that the Ruabon earth referred to is utilised also in the manufacture of tesselated and encaustic tiles. In regard to the composition of earths employed in the manufacture of the commoner kinds of bricks, we may give the following examples:-- _Chemical Composition of Common Brick-earths._ -------------------+-------+-------+------+---------+----------+------ | |Alumina| | | | Water |Silica.| and | Lime.|Magnesia.|Manganese.| and | | Iron. | | | | Loss. +-------+-------+------+---------+----------+------ Reddish-brown | | | | | | brick clay | 52.6 | 30.8 | 3.4 | 2.8 | 1.4 | 9.0 Red-brick clay | 50.4 | 24.0 | 2.7 | 1.3 | --- | 21.6 Common brick-earth | 33.0 | 11.2 | 39.8 | 6.0 | --- | 10.0 Sandy-clay (loam) | 60.2 | 24.0 | 2.4 | 1.6 | --- | 11.8 -------------------+-------+-------+------+---------+----------+------