and in our case this was believed to be undesirable. So it was decided to take only large regular pieces for the erection of a large workmen's dwelling, by which means the application of mortar could be reduced very much. It was a pleasure to see how quickly the building proceeded, and how much more quickly it got dry and habitable than other buildings, where irregular pieces and much mortar had been used. As soon as the workmen and their families began to live in the new building, the traces of damp began to show, and at last the house became the dampest in the whole establishment, and remained so. The thin layers or bands of mortar could not dispose well of the water which was deposited from the air in the house, and this was the worse, as the slag is not like brick and mortar, which suck the water up, but is a vitrified substance, on which water precipitates as on a window-pane.
But how are we to judge, in a given case, whether a house is sufficiently dry? No doubt, in every locality a practical experience establishes itself, founded on the knowledge of the usual material, the manner of building, and the climate. But if, as in some countries, some authority has to declare a house dry and habitable before it is to be let, there will be no end of disputes between this authority and the proprietor, because, after all, apart from the age of the building, the verdict of the experts will be given on their subjective view, not on definite and palpable signs. You know, already, that the absence of damp spots means very little. Feeling by the hand the temperature of the walls, knocking at them with a little hammer, are all of not much good. Not a bad plan is to get from different places in the house small pieces of mortar, and to have them examined as to their contents of evaporable water, which ought not to be more than five per cent, of the weight. But we may have fallen just on dry places only, and get considerably deceived. Direct and comparative hygrometric observations would be best, but the necessary preliminary researches for this kind of examination are still to be made.
But what is to be done if a new building is to be brought quickly and surely into a condition of dryness? I have been obliged to shake your belief in the one means which appeared to exist, the development of carbonic acid by burning charcoal in basins or open stoves. But I shall try to give you something in exchange for what I have taken from you. This something is nothing but an appeal to what we have learned above. There are no means of removing the water from a fresh building but by letting it evaporate into the air. This evaporation, you know, depends on the temperature, the humidity of the air, and its velocity.
Imagine to yourselves a moderately-sized room of 3,530 cubic feet, and the temperature and humidity of the air at the above given mean averages. As one cubic foot of such air is capable of taking up one additional grain of water, the air of the whole room will take up 3,530
