282 JAMES CRAWFORD WATT. 



furnish the best clue to a precise and certain knowledge of the 

 natural one, by showing more clearly how much is due to physical 

 agency " — Rainey. 



part i. microscopic study of precipitation. 

 Technique. 



For simple examination of a reaction, a glass slide measuring 

 38 x 75 mm. was taken and on it were outlined two squares with 

 sides of 20 mm. painted in melted paraffin, so that the wax formed 

 the sides of a shallow cell. Into a cell were placed a small amount 

 of the two reacting solutions to give the desired precipitate, and 

 then a cover glass 22 mm. square was dropped on, supported by 

 the wax cell wall. If it was desired to keep the specimen, melted 

 wax was then painted all around the edges, and overlapping on to 

 the cover glass. The wax was then coated with thick shellac, 

 which in its turn overlapped slightly on to the glass. The cell was 

 thus permanently sealed. Two cells could be made on one slide 

 and the same, reaction could be compared in distilled water and in 

 a colloidal solution side by side. 



It was found desirable to examine not only the precipitate formed 

 immediately at the line of contact of the two reacting solutions, but 

 also what was formed later, in more remote parts of the solution, 

 during diffusion, and as the concentration of the dissolved salts 

 became less and less with increasing precipitation. To do this 

 required larger cells, and it was desirable to have a fairly uniform 

 thickness of cell, which was accomplished as follows : 



On a glass slide 38 x 75 mm. in size two thin parallel strips of 

 mica dipped in melted paraffin were so placed as to support the 

 long edges of a glass cover slip measuring 24 x 50 mm., which was 

 placed upon them. These long edges were now thoroughly painted 

 over with melted paraffin, which on hardening cemented the cover 

 slip to the slide. A cell measuring from 0.1 mm. to 0.2 mm. deep 

 was thus made, still open at both ends. By means of a fine glass 

 pipette one of the reacting solutions, M/10 calcium chloride, was 

 now introduced at one end until the cell was nearly two thirds 

 filled. This end was then carefully dried and sealed with paraffin. 



