The student should always acquaint himself with the cost of the materials he uses in a preparation. He should therefore consult the price list of some well-known manufacturer or retailer. Such a list should be available in every laboratory. Having ascertained such prices, he should always, after completing his preparation, calculate the cost of his final product, and compare with the price in the price list. Many factors control the market prices, such as labour, recovery of by-products, etc. ; it is, nevertheless, a good exercise to make this comparison. In deciding which particular method of preparation should be adopted in a particular case, the student should consider the question of cost, weighing up the cost of starting materials, reagents, solvents, with yield. For example, there is a well-known reaction by which aromatic amines are converted into the corresponding hydrocarbons. It would be little short of a crime, however, except in order to confirm the reaction, to attempt to prepare toluene from toluidine in this way, since the value of the final product is only a fraction of that of the starting material, apart altogether from the cost of the reagents required ; often such reactions can well be studied in a test-tube.