Section 12. The department shall make, free of charge, a chemical analysis of any narcotic drug, or any synthetic substitute for the same, or any preparation containing the same, or any salt or compound thereof, and of any poison, drug, medicine or chemical, when submitted to it by police authorities or by such incorporated charitable organizations in the commonwealth, as the department shall approve for this purpose; provided, that it is satisfied that the analysis is to be used for the enforcement of law.