Identification of Organic Compounds. 25 



first employed by Liebig. With the certainty of being 

 able to ascertain with accuracy and readiness the com- 

 position of a compound, the progress of organic chemistry 

 made rapid advances. 



NECESSARY PRELIMINARY PRECAUTIONS. 



It cannot be too often reiterated that it is generally 

 useless to submit a product to quantitative analysis until 

 the investigator has satisfied himself as to its purity and 

 freedom from admixture with other substances. Two 

 criteria of purity, namely, melting and boiling points, have 

 already been discussed in some detail. It happens, how- 

 ever, that solid substances, even when they exhibit a sharp 

 melting point, often contain appreciable quantities of the 

 solvent from which they have been recrystallized, which is 

 sometimes combined with them in molecular proportions 

 in the form of solvent of crystallization. It is usual to 

 eliminate this before submitting a substance to analysis, 

 or determining finally its melting point. The solvent-free 

 substance will usually have a different melting point to 

 that containing solvent of crystallization (which is often 

 quite a definite one, and does not alter on recrystallization 

 from that solvent), and the fact as to whether the latter is 

 absent or present should always be stated when a melting 

 point is given. 



The adhering solvent can be eliminated in various 

 ways. Often a substance can be obtained quite solvent 

 free on keeping in air ; more often it is necessary to 

 eliminate the solvent by heating (to volatilize it) the sub- 

 stance, or by keeping it in vacuo (especially when the 

 substance decomposes readily) over some material, such 

 as concentrated sulphuric acid, which readily combines 

 with the vapour. In other cases, when the solvent is 



