The life of the “scum” is very variable; sometimes, after having once appeared and disappeared, it will never come again. The passing shower may wash it off (though it is not always so easily removed), and it may come again and again for years. It behaves very erratically. The amount of the efflorescence may be such as, in course of time, to lead to the surface of the brick “bursting” and peeling off, or, on the other hand, it may be a mere film. There is one thing in connexion with efflorescence which cannot be overlooked in regarding its practical effects in the building. In ever so many cases we find that the scum, or the major part of it, is only to be found in the neighbourhood of the mortar joints. That is a matter of direct observation, and we have taken some considerable trouble to verify it, as it has always been regarded as a point whereon to hinge a debate. We do not say that in all cases the efflorescence appears only in the position on the brick just indicated; but it unquestionably does so in too many instances to enable us to regard its occurrence as mere accident. Taking a large surface of brickwork just commencing to show efflorescence, we find that the vicinity of the mortar joints are the first places, in very many instances, where the nuisance begins to manifest itself. From thence it spreads over the surface of the brick until the whole is more or less discoloured. It seems impossible to deny that the mortar is guilty, to some extent, in such cases. At the same time, we must confess that we have never seen the efflorescence spreading over the mortar. It would appear that something in the mortar enters into chemical alliance with certain ingredients of the brick, and that neither without the other could produce the phenomenon alluded to. The remedy suggesting itself most readily is to chemically analyse the efflorescence, the brick, and the mortar; supplementing the experiments with a micro-examination to see how far it is possible to locate the deleterious substances found to exist, so that they may be removed in the manufacture of the materials, if that is possible. But information on that head is of the scantiest description, and much more will have to be done before the question is definitely settled. Another kind of “efflorescence” that often appears on bricks in damp situations is mere vegetable growth, which bears a superficial resemblance to the crystalline “scum” just described, though it can, of course, be easily differentiated on examination with a lens. The damp atmosphere is no doubt largely responsible for this, though ineffectual damp-courses are contributors. The remedy lies in having a less absorbent brick--one that will not afford ready foothold to the vegetation.