In general, we have to consider whether the brick is strong enough for the purpose to which it is to be applied; and that depends much more on the manner in which it is built up, than on the strength of the individual brick. For ordinary building purposes almost any kind of brick is, _per se_, strong enough, and a mere inspection of the specimen suffices to carry conviction as to its suitability or otherwise in that respect. For certain structures, such as buildings to carry heavy weights--especially moving weight--for engineering purposes, and the like, we ought, it is true, to know a little more. Yet the engineer would be a very poor one who could not tell at sight whether a brick submitted to him was fit or not for the purpose he has in view, from the point of view of its weight-carrying properties. In any case, however, fashion demands the “crushing weight” in figures, and although such figures are in general of but little practical value, they must be given. The principal difficulty the architect and engineer have to contend with is not lack of strength, but the setting in of decay, and that even in bricks sometimes of the strongest description. Unless the strength is going to be maintained, it is of no use whatever, in a scientific sense, to give it in the first instance. After these few preliminary observations, it will be well to treat the subject more systematically. THE EFFECT OF THE ATMOSPHERE ON BRICKS. Air is a mixture of gases; dry air consists of at least four of them, namely, nitrogen, oxygen, carbonic acid, and argon. Of these, by far the most abundant is nitrogen, present to the extent of about 78 per cent., then oxygen, 20.96 per cent., argon about 1 per cent., and carbonic acid 0.04 per cent. Extremely minute quantities of ammonia and ozone, though practically always present, have been omitted from the preceding results of analysis of air. We have been speaking of pure dry air; but the atmosphere is hardly ever of precisely the same chemical composition in two different places. By the seaside it has more ozone, and chloride of sodium is found in particular abundance. In cities, especially where large factories exist, nitric acid and sulphuric acid appear most conspicuously, and the proportion of ammonia becomes larger. In the air of streets and houses, the proportion of oxygen diminishes, whilst that of carbonic acid increases. Dr. Angus Smith has shown that very pure air should contain not less than 20.99 per cent. of oxygen, with 0.030 of carbonic acid; but he found impure air in Manchester to have only 20.21 of oxygen, whilst the proportion of carbonic acid in that city during fogs was ascertained to rise sometimes to 0.0679, and in the pit of a theatre to the very large amount of 0.2734. Although these may seem to be very small percentages, yet the total amount of carbonic acid in the atmosphere is enormous, and plays a conspicuous part in the decay of certain kinds of bricks.