"No. 104.] 217 



Caen stone of Normandy, and some of the less coherent limestones of 

 modern geological formations are strong enough and quite durable for 

 buildings in Southern Europe or where the frosts are not extreme ; 

 but in a climate like our own, they are rapidly destroyed by the alter- 

 nate action of freezing and thawing. 



Some of the finer sandstones, which have a considerable amount of 

 argillaceous matter, are perfectly capable of withstanding moderate 

 freezing ; but the extreme changes from a moist condition, or one sat- 

 urated with moisture, to the extreme of freezing, are fatal to their 

 durability. 



As before repeated, any stone in which clay enters largely, or a 

 porous stone of any kind, is liable to decay under the extremes of wet 

 and frost. The penetration of moisture among the particles of the 

 stone, and its expansion on freezing, destroy the cohesion of the parts, 

 and the succeeding rains wash away the loosened particles. In this 

 way, during a long succession of years, the surface is disintegrated and 

 the structure gradually crumbles. Although some stones are more 

 susceptible to these atmospheric influences than others, yet none are 

 entirely free from its effects. 



Even the changes of temperature, without frost or moisture, operate 

 upon the masses of stone and cause a motion of the particles. The 

 observations of Prof. Horsford upon the pendulum suspended within 

 the Bunker-hill Monument show that this massive structure *'is 

 scarcely for a moment in a state of rest, but is constantly working and 

 heaving under the influence of the every varying temperature of its 

 different sides." When to this is added the extreme action of freezing 

 and thawing, it cannot be surprising that the poorer materials will 

 fall into dilapidation, or that the beSt selected building-stone will ulti- 

 mately give way. This cause operating everywhere, at all times and 

 through all seasons, is a far more active agent in the destruction of 

 buildings than all the others operating together; and though it may 

 sometimes require years for an appreciable change to be accomplislied 

 upon a sound material, it is nevertheless constantly going on, however 

 slow the change mav be. 



2. The imprope?' laying of stone by presenting the faces of laminse 

 to the weather, often hastens the disintegration of the mass. I have 

 already alluded to this especially in regard to the brown freestone 

 which is now so extensively used, and which presents such uneven 

 weathering, from being in part laid according to the bedding, and in 

 part with the bed facing the exterior. 



3. The vegetation of microscoipic lichens takes place upon the surface 

 of the stone, when, from any cause, that surface becomes roughened so 

 as to afford a lodgment for the seeds or spores of these plants. These 

 growing, still further hasten the disintegration of the stone, and accu- 

 mulating about them the fine dust floated by the atmosphere, become 

 points for the absorption of more water, which on freezing still further 

 roughens the surface, and the patch of lichen gradually extends. These 

 lichens often gain attachment upon the surface of a finely dressed 

 stone, from some little inequality of texture, or from softer material 

 that more readily becomes decomposed, or more readily accommodates 

 the growth of the plant. Such stones in time become partially or 

 entirely covered by lichens, and present an unsightly aspect. Thei 



[Assem. Doc. No. 104.] 38 



