254 THE POPULAR SCIENCE MONTHLY. 



slacking the fires, and allowing the distillation to proceed so slowly 

 that the dense portions of the vapors are condensed on the dome of 

 the still, and, falling back upon the surface of the hot oil, are heated 

 above their boiling-points, and decomposed into a lighter oil and a car- 

 bonaceous residue. By continuing this process for several hours the 

 oil has passed out of the still, leaving a quantity of residuum, as in 

 the first instance. This cracking process is never complete, as a por- 

 tion of the oil is cracked too much and another portion too little ; but 

 the average gives a burning-oil of the proper density, color, etc., while 

 in other respects it is greatly inferior to the oil that is not cracked. 

 The reader will readily perceive that by mixing these constituent oils 

 of the petroleum suitable for burning-oil, which have been very prop- 

 erly called " normal " burning-oils, with different proportions of the 

 cracked oils, a great variety of products may be obtained ; but I pro- 

 pose in this article to speak of only three classes of burning-oils, and 

 to show that these three classes may furnish oils that meet the de- 

 mands of legal enactments, while at the same time they may be both 

 very dangerous and very bad. 



The first class of oils mentioned that are distilled from the petro- 

 leum unchanged consists of compounds of hydrogen and carbon com- 

 bined in such proportions that the percentage of hydrogen is greater 

 than in any other similar substances. In addition, they are very inert 

 to chemical reagents, in this respect resembling paraffine or India-rub- 

 ber. They may be washed with sulphuric acid or strong solution of 

 caustic soda, and very completely purified ; but they are not acted on 

 by either of these powerful reagents, and the product is a pure, color- 

 less oil, with the odor of kerosene, and burning with a dazzling, white 

 flame. The wick is burned but little more rapidly than that of an al- 

 cohol-lamp. The flame does not smoke, neither does it emit any un- 

 pleasant odor. These oils are safe, healthful, and economical ; in fact, 

 they constitute the best and cheapest illuminating agent ever given to 

 man. 



When the oils too heavy for illuminating oils are destructively 

 distilled or " cracked," the product is largely contaminated with oils 

 containing less hydrogen in proportion to the carbon, and which are 

 not inert to chemical reagents like those just described. When these 

 oils are treated with sulphuric acid, both the oil and the acid are de- 

 composed. The sulphur and a part of the oxygen of the acid (S0 2 ) 

 take the place of a part of the hydrogen of the oil, while this hydro- 

 gen unites with the remaining oxygen of the acid and forms water. 

 This sulphur and oxygen thus become constituents of the oil, and when 

 the oil is burned they escape into the room as sulphurous oxide iden- 

 tical with the fumes of burned brimstone. But this is not all : the sul- 

 phur compounds, and the heavy, imperfectly cracked oils, soon impair 

 the capillary attraction of the wick ; and, the flow of the oil being im- 

 peded, the w r ick becomes charred and coated with unburned carbon. 



