PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 555 



fusible, but that is not so; china is more fusible. The colors 

 which are given to pottery are taken from metallic oxides. He 

 did not think that the analysis of the different kinds of pottery 

 presented by Mr. Johnson, chemically speaking, represented them 

 properly. 



The Chairman (to Dr. Stevens.)— Can you give us any geologi- 

 cal reason why the bricks become paler as you leave the sea-board 

 and come nearer Milwaukie ? They are nearly pale v^^hen we get 

 there. 



Dr. Stevens. — Because there is not so much iron in them. The 

 clay of which the Milwaukie bricks are manufactured is composed 

 of Debonia strata. 



The Chairman (to Mr. Seely.) — Would it be expected by a 

 chemist that a brick pressed out of a mass and burnt would be a 

 tenacious brick ? 



Mr. Seely. — It would. 



The Chairman said he could take one of those bricks so made 

 and break it to pieces with a cane. 



Mr. Johnson stated that he saw bricks made in Poughkeepsie, 

 which yielded on the pressure being removed. 



Mr. Garvey did not think that the finish of a brick depended 

 on the pressure. He found in the ordinary hand brick that it 

 was an advantage to make them porous; also, that if the outer 

 surface of a brick is more burnt than the next stratum it would 

 not be homogeneous, and would flake off. 



The Chairman. — When I break a well made brick, composed 

 of a mixture of clay and sand, it presents a glassy surface where 

 broken. That is not the case in one made by pressure. 



Mr. Dibben. — Dry clay bricks will never make a good wall to 

 stand the effect of the Aveather. 



The Chairman. — The building belonging to the society of the 

 Deaf and Dumb Association has been built of Milwaukie bricks, 

 and I have found that the north side of it, where the sun does 

 not shine, has become quite green. Is this a vegetable growth ? 

 Is it there because of the absence of iron ? 



Mr. Johnson said the same could be observed on Trinity build- 

 ing. 



Mr. Garvey did not think the composition of the clay had 

 anything to do with it. He thought the moisture was the occa- 

 sion of it. 



Mr. Johnson said that some samples of pottery which were on 



