If White to move, he can easily manoeuvre so as to bring about the same or a similar position with Black to move. For instance
1 Q-Q4ch.
K-R8;
2 Q-R8ch.
K-Kt8;
3 Q-R4.
It is not difficult to multiply researches of the above type concerning the exchange-value, and such exercises are of exceeding use to the student. What has been said is sufficient to guide him in this work, which I should recommend to him most earnestly.