IT is difficult to give at the outset a clear and concise idea of what is understood as chemical science. The best and simplest definition of chemistry is a modification of that given by Webster: That branch of science which treats of the composition of substances, their changes in composition, and the laws governing such changes. It will be seen that the essential character of the science is that it has to deal with composi tion, and in this the line between physical and chemical phenomena is more sharply drawn than that between the individual varieties of the former. A bar of soft iron is the same in composition, whether it be hot or cold, luminous or non-luminous, magnetized or not magnetized. When, however, it comes under the dominion of chemical action, its composition is changed, and, although the resulting substance contains iron, it differs in its appearance and properties from that metal. Moreover, this change in composition, once brought about, is permanent until another change is wrought by another manifestation of chemical action; on the other hand, the peculiar property communicated to a substance by a physical force is temporary, and only manifested during the action of that force.