The third change is the omission of the laboratory directions, these having been found unsuitable. The needs of different teachers differ so widely that it is impossible to draw up a scheme that shall answer for all. In place of the laboratory directions for students we have therefore given, in an appendix, a series of prae tical suggestions to teachers, leaving it to them to work out de tailed directions, if desired, by the help of the standard labora tory manuals. These suggestions are the result of a good deal of experience on the part of many teachers besides ourselves, and we hope they will be found useful in procuring and preparing material (often a matter of considerable difficulty), and in decid ing just what the student may reasonably be expected to do. For the rest, the original matter has been thoroughly revised.