The lumps at the top of the pile are least exposed to the heat, and very often still contain carbonate, as is shown by the effervescence produced on treatment with an acid. Such lime is imperfectly burnt, and the lumps frequently still exhibit the crystalline structure of limestone when broken. They quench rapidly, but when mixed with a little extra water, the mass is no longer of the buttery consistency typical of caustic lime, but contains gritty portions consisting of unaltered limestone. Owing to the defects of dead burning on the one hand and insufficient calcining on the other, colour-makers now prefer lime that has been burned in continuous kilns, because, when properly made, such lime is very uniform in character, and is also cheaper than that burned with such an expensive fuel as wood. In consequence of the greater capacity of the continuous kiln, and the more uniform character of the product, the old-fashioned kilns are more and more falling into disuse. The arrangement of the continuous kiln is very simple. The kiln consists of a fairly high shaft, open at the top, and provided at the bottom with a small hole for the removal of the burnt lime. A coal fire is lighted, and as soon as the kiln is heated up, alternate layers of limestone and sufficient coal for burning it are introduced. The burnt lime sinks to the bottom of the shaft and is pulled out, with iron hooks, from time to time. Given the right proportions of coal and limestone, the lime made in these kilns is burnt to just the right degree, and is excellent for builders’ use. In many cases, however, it is less valuable to the colour-maker, and in some quite useless. For example, when the coal is not completely consumed, carbon, even though only a very small quantity, is deposited on the lime, and the burnt lime, instead of being a brilliant white, as it should be, is grey; and colour made therefrom is also greyish white and will spoil the shade of other colours with which it is mixed. The chemical composition of the original limestone also has an influence on the character of the burnt lime. Limestone consisting entirely of carbon dioxide and lime is so rare that sufficient is never available for making burnt lime on a large scale. Even the purest limestone found native in large quantities--namely marble--is not pure carbonate of lime, but contains a certain proportion of extraneous substances. At the same time it is too expensive to use for technical purposes. The ordinary impurities present in limestone are ferrous oxide, ferric oxide, magnesia and organic matter. The presence of ferrous oxide can usually be detected by the greenish tinge of the raw limestone, and the reddish cast of the burnt product. Ferric oxide is revealed by its reddish colour, in both the limestone and burnt lime.