This is one of the most important parts of our subject, and it may be approached from several points of view. When a brick decays, its structure, for the most part, is responsible therefor. A great deal depends on whether the ingredients forming the brick are merely baked in the process of manufacture, or whether they are wholly or in part agglutinated by igneous fusion. A rough and ready plan of determining this point, in the absence of experience, is by ascertaining the porosity of the brick. Other things being equal, the absorption test is undoubtedly the best all-round method of gauging the weathering qualities of a brick. But there are certain kinds of bricks which defy that method; an imperfectly burnt one with a vitreous exterior is especially treacherous in that respect, and, indeed all “vitrified” bricks are difficult to deal with by the “absorption process.” Again, a brick cracked all over, not with superficial cracks only, but with those which go far into the interior, will not yield its quality by mere immersion in water. The water, it is true, finds its way right into the brick, but, as often as not, the sides of the cracks are perfectly vitrified and almost damp proof, so that on lifting the brick out of the water the latter rolls off as though it were on “a duck’s back.” Yet such a brick, yielding but the merest fraction as a result of the immersion, may be utterly worthless when put into a building, because it would not be strong enough. Then we have those bricks which are seriously affected chemically, but which seem fairly good in other respects. They also, in many cases, defy the efforts of the experimenter in regard to absorption; though they are nevertheless easily detected as being of bad quality, by other methods. Such bricks often resist great “crushing weights,” and generally bear a good character, their subsequent behaviour when put in the building to the contrary notwithstanding. In determining the weather-resisting qualities of a brick we have the following things to consider:-- 1. The chemical composition of the brick. 2. Its absorptive capacity. 3. Its minute structure. 4. Its specific gravity. 5. Its strength. The last-mentioned property can often be inferred from a knowledge of the three preceding ones, and need not, therefore, form the subject of direct experiment. In spite of that, however, we find that the “crushing strength” is much more popular than the others. The reason, so far as brick manufacturers are concerned, is not far to seek. Architects demand that especial quality. “What is the ‘crushing strength’ of your bricks?” enquires the architect. And if the maker does not know, he stands a good chance of losing the order. Figures are demanded, and if the maker cannot produce a higher figure than his neighbour, woe betide him. But statistics are ever deceptive, and as applied to bricks in regard to their strength especially so.