484 



Chronicles of Science. 



[July, 



experiment. Over one arm of the tube is fixed a porous plate. A 

 superior material for the porous plate has been found in the artificially- 

 compressed graphite of Mr. Brockeden. It is sold for making pencils 



Fig. 1. 



in small two-inch cubes. This is easily cut into slices by means of a 

 steel spring saw, and by rubbing it on a piece of flat sandstone, it 

 may be reduced to any degree of thinness. A circular disc of this 

 substance, about the size of a wafer, is placed over one arm of the 

 U tube. This is carried into the colliery, and the percentage of gas 

 in the air is shown by the rise of the fluid in the open arm of the tube. 

 Fig. 4 is a section of a porous cell to be used with the apparatus 

 shown in Fig. 3. Fig. 5 consists of (a) a porous tile secured into a 

 glass vessel (b), which is connected with a U tube having some 

 mercury in it. The arrangement is then made as in the former case, 

 so that the index is moved the moment diffusion takes place. 



