Proceedings of the Polytechnic Association. 763 



country, and tliat many amateur microscopists vrlio, until lately, 

 j)rided themselves upon the possession of European microscopes, have, 

 in order to see the most minute revelations lately made, been obliged 

 to substitute objectives made in America. 



Yaiueties of Cokstelx'tive Material. 

 Professor Ansted, F. E. S., in a papei- read before the Royal Insti- 

 tute of British Architects, '•• on the Relations of Geology to Archi- 

 tecture," has presented many points of practical bearing. After 

 speaking of the origin of all rocks yielding constructive material, 

 which may be grouped under three distinct heads, as acpieous, igne- 

 ous, or metamorphic, he said the mineral composition of rocks is in 

 some cases very simple and evident ; in other cases moderaiely com- 

 plicated, but easily recognized ; in others, again, so complex as to 

 need long experience or chemical analysis to determine its nature. 

 As the enduring power of stone, when exposed to certain influences, 

 depends, in some measure, on mineral composition, this subject is 

 one to which the architect should direct attention. A large propor- 

 tion of the building material in common use is limestone, more or 

 less pure, and the rest is chiefly sandstone. In the former case car- 

 bonate of lime, and in the latter silica is the chief ingredient. Clays 

 mialtered are only used when manufactured into bricks, and the sul- 

 phates of lime (gypsum and alabaster) are chiefly adapted for internal 

 use or small ornaments. The granites and porphyries are limited to spe- 

 cial uses, owing to their extreme hardness. The varieties of marljle may 

 be regarded as varieties of limestone. Slates and slabs, very useful in 

 architecture, and forming a special class of rocks due to metamorphic 

 action, are derived from clay. Limestones are simple carbonates of lime, 

 mixed carbonates of lime and magnesia (serpentines),or sidphates of lime. 

 According to their texture, to the cementiiig medium in the case of 

 the bedded limestones, and in others to the extent of crystalline action 

 or metamorphosis they have undergone, they ar.e available for special 

 purposes. They owe their color to metallic oxyds. When semi- 

 crystallized, and having a grain too flne to be recognized, they are 

 niarbles of the common kind, gypsum, or serpentine, according to 

 their chemical composition ; when perfectly crystallized, they are statu- 

 ary marbles or alabaster ; when the grain can be recognized, they are 

 freestones ; when the grain of the stone is mixed with fragnients of 

 shell, they are rag stones, or shell of limestones ; when the grain is 

 formed of a multitude of round concentric particles, like the roe of 



