ON A PROCESS OF FRACTIONAL CONDENSATION. 131 



bodies are present, even in considerable proportion, a much larger quantity would be 

 required than if the material were but slightly volatile; as the waste in the former 

 case, from evaporation, would be much greater. 



But in many cases it will be found that highly volatile bodies are present only in 

 very small proportion, — e. g. in viscid petroleums like Rangoon tar, and in the pro- 

 ducts of distillation of some species of asphalt. In such cases, the requisite quantity 

 to be operated upon, to obtain the most volatile constituents in sufficient quantity for 

 anything like a complete study of their chemical relations, would be extremely large, 



too large to be conducted in the laboratory, — and one would have to resort to the 

 manufactory for the first distillation. I have dwelt at some length on this point, hav- 

 ing experienced the disappointment which one feels, after months of labor, on finding 

 the products insufficient for his requirements, when the expenditure of a little more 

 time, comparatively, might have given double the quantities obtained. 



In the first series of fractioning I generally operate on successive portions, of about 

 one gallon each, of the crude material, and take off a fraction for every 20° C. rise of 

 temperature of the retort. These fractions are preserved in well-stoppered bottles, 

 and each carefully labelled with the temperatures between which it was obtained. 

 The fractions for each fresh portion of the crude material, being collected between the 

 same limits of temperature, are added to the corresponding products from the preced- 

 ing operations, till enough of the crude material has been taken to insure, ultimately, 



a sufficiency of the pure products. 



In the commencement, not only of this but of all subsequent fractionings, when the 

 temperature to which the bath should be raised is unknown, I first bring the liquid in 

 the retort into full ebullition, so that a steady stream of liquid shall flow back from 

 the end of the worm into the retort. I then carefully raise the temperature of the 

 bath until the vapors from the retort pass through the heated worm so freely that the 

 liquid, in condensing from them, shall drop with tolerable rapidity into the cold 



receiver. In order that this dropping may b 



necessary that th 



temperature of the bath should rise very gradually as the more volatile constituents of 

 the mixture are taken off; this is easily effected by carefully regulating the flame 



under the bath. 



It is advisable to boil the retort as rapidly as possible without choking the lower 

 end of the heated worm with the returning liquid. As this choking would give rise 

 to additional pressure in the retort, and consequently occasion abnormal elevation of 

 the temperature, and possibly a rush of liquid into the receiver, and thus introduce 

 irregularities in the work, excessive heat under the retort should be avoided. The 



