Whilst speaking of the various acids which attack and destroy bricks, we must not forget those formed by the decomposition of organic matter on the surface of bricks which “vegetate.” The lichens, mosses, and so forth, growing from cracks in the wall, or spread over on to the brick from the mortar, yield, on decomposition, some of the most powerful acids in existence. A brick with a “crumbly” surface affords good foothold for these plants, and when they die they give rise to the so-called humus acids--crenic and apocrenic acid--which undoubtedly do an immense amount of damage. By keeping the surface of the brick moist, the plants permit the ordinary acids in rain to do more execution than they otherwise would. Taking two bricks, one which “vegetates” and one that does not, and exposing them in the same situation, it will be found that after a smart shower of rain the surface of the former has become thoroughly soaked, and the vegetation keeps it so, completely rotting it in time; whereas the surface of the latter, exposed to the same shower, may be quite dry within an hour or two after the rain has fallen. Returning to the subject of rainfall, which exercises such material influence on the durability of bricks, we may give a few particulars concerning the distribution of rain in this country. Speaking generally, the east coast of England is the driest part of the country, the west coast having the greatest rainfall. The annual quantity at sea-level ranges from 60 to 80 inches on the west coasts of Ireland and Scotland, to about 20 inches on the east coast of England.[10] In some localities, however, the fall is much greater, amounting to 154 inches on the average of six years at Seathwaite, in Borrowdale, at the height of 422 feet above the sea. The quantities which fall in particular showers are often very great, and this aspect of rainfall also has its interest for us. About London a fall exceeding an inch in 24 hours is comparatively rare, although on August 1, 1846, 3.12 inches were collected in St. Paul’s Churchyard in two hours and seventeen minutes.[11] On our west coasts this amount is often exceeded. On October 24, 1849, 4.37 inches were collected at Wastdale Head; June 30, 1881, 4.80 inches at Seathwaite; on April 13, 1878, 4.6 inches fell at Haverstock Hill, London; and a fall of 5.36 inches was recorded from Monmouthshire on the 14th July, 1875. Taking averages of districts, we may give the following statistics, referring, of course, to annual rainfall:--