516 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



Don, whether originally cast upon the gun or afterwards turned, 

 will invariably, after the gun has been fired a few hundred times, 

 present centres of crystallization ; and that in this respect turn- 

 ing these angles has no advantage. It seems that a change is 

 going on in the forms of the crystals of the iron, during the 

 vibrations, resulting from the explosion of the gunpowder within 

 the gun, and especially at the angles, so that these angles are 

 invariably the points at which fracture takes place. It is for 

 this reason that in the army we have now cast aside all the old 

 ornaments, and resorted to the smooth plain gun. Capt. Dahl- 

 green, finding that casting trunnions upon the gun weakened them, 

 went so far as to cast a gun large enough to take in the trun- 

 nions, and then turn it down ; but he found that it had no effect 

 upon the crystallization. He, therefore, now casts his trunnions 

 on a separate piece, with a band to pass around the end of the 

 gun to receive the recoil. According to these experiments, 

 therefore, nothing would be gained by turning the grooves in the 

 gun as suggested. 



Mr. Dibben said that the same law had been known before 

 Capt.Dahlgreen's experiments, although not so fully demonstrated. 

 But it is not merely after use, but at the time of casting ; if there 

 is unequal thickness there will.be unequal crystallization. The 

 Parrott gun is the best now in use, and that is made by shrinking 

 wrought iron over the breach of a cast iron gun. 



Dr. Stevens suggested that thus far most of the large guns had 

 exploded before they had hurt anybody. 



COAL OILS AND PETROLEUM. 



Mr. C. W. Smith said that the coal oils are made up of sub- 

 stances very different in their characters. 



The first product of their distillation is naptha. This has been 

 used as a substitute for turpentine. An anaesthetic has also been 

 extracted from it. Coal oils are now extensively used for illumi- 

 nation, and are going into extensive use for lubrication, being 

 found for that purpose superior to the best sperm oil. An oil 

 has been prepared from petroleum, suitable for use upon leather, 

 as a tanner's oil, and various other uses have been found for 

 these oils. They may be found serviceable for fuel for steam- 

 ships. 



Dr. Stevens. — It is true that coal oil, understanding by that 

 term not only the hydrocarbons derived from the distillation of 



