Dr William Craig of Glasgow, in a letter to Mr Graham of London, published in the 42d vol. of the _Medical and Surgical Journal of Edinburgh_, states most interesting facts connected with this subject, particularly in regard to black matter found in the pulmonary structure of old people, which deserve considerable attention. He says--"I found that a black discoloration of the lungs was by no means a rare occurrence amongst those old people; and that it was impossible in many instances to decide, whether the black colour was owing to an increase of what is called the healthy black matter,--to a morbid secretion, or to a foreign substance being imbedded with the atmospheric air. After examining a considerable number of lungs, and finding that the division of the black matter into three kinds was not founded upon observation, and that the descriptions of them given by the best authorities were insufficient to enable us to distinguish them from one another, I begin to think, that in every instance in which black matter is found in the lungs, it ought to be considered morbid. If we examine the lungs at different stages of life, we find as a general rule that the quantity of black matter increases with age. In young children we find no traces of it, the lungs being of a reddish colour. At the age of ten years the black matter makes its appearance in the outer surface of the lungs, and in the interlobular spaces. At the age of thirty or forty, the lung presents a greyish or mottled appearance, and the bronchial glands contain more or less black matter. Between the age of seventy and a hundred, the lungs are generally infiltrated with fluid black matter, which can be expressed from the cut surfaces, and stain the hands black." "There are many circumstances which favour the accumulation of this black matter in the lungs; for instance, long-continued living in a smoky atmosphere, like that of this city, the inhalation of coal-dust, as in the case of colliers, or of charcoal-powder, as in the case of iron-founders. There can be no doubt that we inhale foreign substances along with the atmospheric air.