Portland cement is .generally received in sacks in this country, in casks abroad; these should at once be emptied,the cement spread out for a month or so on a wooden or concrete floor, to a depth not exceeding 3 or 4 feet, in a large weather-tight room, and occasionally turned over, so that it may become thoroughly air-slaked and cooled. (See tests for coolness, p. 174.) During the time it is thus exposed the cement if fresh will increase considerably in bulk.1

Sometimes cement which if tested when new would crack or "blow," will be found after this cooling to have lost the tendency to do so.

This air-slaking or "cooling" is of the very greatest importance, particularly with cements which, on account of their very high tensile resistance, may be suspected of containing an excess of lime.