204 THE POPULAR SCIENCE MONTHLY. 



and in our case this was believed to be undesirable. So it was de- 

 cided 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 habi- 

 table 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, ou which water pre- 

 cipitates as on a window-pane. 



But how are we to judge, in a given case, whether a house is suf- 

 ficiently dry ? No doubt, in every locality a practical expei-ience 

 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 declai'e 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 difierent 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 hygro- 

 metric observations would be best, but the necessary preliminary re- 

 searches 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 Ave have 

 learned above. There are no means of removing the water from a 

 fresh building but by letting it evaporate into the air. This evapora- 

 tion, 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 



