NATUKAL PHILOSOPHY. 199 



every accession of moisture and lengthened as the air becomes too dry, it fol- 

 lows that the cock is alternately opened and closed, according as the air in the 

 building requires more or less moisture. 



ACCURACY OF THE CHINESE AS OBSERVERS. 



At a recent meeting of the Chinese Asiatic Society, Dr. MacGowan remarked 

 that the subject of Snow Crystals, which engaged considerable attention last 

 winter hi England, had come under the notice of Chinese observers ; that from 

 a remote period there has been an axiomatic expression met with in their 

 conversation and books, to the effect that " snow-flakes are hexagonal, " which, 

 like their knowledge of the acarus in the cuticle of persons affected with itch 

 (figured and described by their medical writers long before Western patholo- 

 gists admitted the existence of that parasite), shows, among many similar 

 cases, that the Chinese are accurate observers of nature. 



ON THE STRENGTH OF BUILDING MATERIALS. 



At the Providence meeting of the American Association Professor Henry read 

 a paper on the modes of testing building materials, and an account of the mar- 

 bles used at Washington. He had been appointed on a committee to test the 

 materials offered for the extension of the Capitol at Washington. The com- 

 mittee had to take into consideration many minute sources of disintegration, 

 such as that every flash of lightning produced an appreciable amount of nitric 

 acid, which diffused in rain-water acted on the carbonate of lime and the ac- 

 tion of dust earned by the wind against the building. The committee sub- 

 jected specimens to actual freezing, and after several experiments a good 

 method was obtained. It was found that hi ten thousand years one inch 

 would be worn from the blocks by the action of frost. Blocks of one and a 

 half inch cube were subjected to pressure, and thin plates of lead, as had been 

 the case in former experiments, being introduced to equalize any inequalities 

 which might occur in the surfaces. But upon experiment it was found that 

 while one of these cubes would sustain 60,000 pounds without the lead plates 

 it would sustain only 30,000 with them. They had therefore to invent a ma- 

 chine to cut the sides of the block perfectly parallel, when it was found that 

 the marble which was chosen for the Capitol, from a quarry in Lee, Massa- 

 chusetts, would sustain about 25,000 pounds to the square inch. The manner 

 of its breaking was peculiar. With the lead plates interposed, the sides, 

 which were free, first gave way, leaving the pressure on two cones whose 

 bases joined the plates, and whose apexes met each other, and that they then 

 yielded with comparative ease. This marble absorbed water by capillary at- 

 traction, and in common with other marbles was permeable to gases. Soon 

 after the workmen commenced placing it in the walls it exhibited a brownish 

 discoloration, although no trace of it appeared while the blocks remained in 

 the stone-cutter's yard. A variety of experiments were made with a view to 

 ascertain the cause of this phenomena, and it was finally concluded to be due 

 to the previous absorption by the marble of water holding in solution organic 



